Remove support for Python 2.6 (#27256)

Modifications:
- [x] Removed `centos:6` unit test, adjusted vermin checks
- [x] Removed backport of `collections.OrderedDict`
- [x] Removed backport of `functools.total_ordering`
- [x] Removed Python 2.6 specific skip markers in unit tests
- [x] Fixed a few minor Python 2.6 related TODOs in code

Updating the vendored dependencies will be done in separate PRs

27 files changed, 66 insertions, 327 deletions

diff --git a/.github/workflows/unit_tests.yaml b/.github/workflows/unit_tests.yaml
index 2e8f9deef6..585271e04c 100644
--- a/.github/workflows/unit_tests.yaml
+++ b/.github/workflows/unit_tests.yaml
@@ -24,9 +24,9 @@ jobs:
         pip install --upgrade pip
         pip install --upgrade vermin
     - name: vermin (Spack's Core)
-      run: vermin --backport argparse --violations --backport typing -t=2.6- -t=3.5- -vvv lib/spack/spack/ lib/spack/llnl/ bin/
+      run: vermin --backport argparse --violations --backport typing -t=2.7- -t=3.5- -vvv lib/spack/spack/ lib/spack/llnl/ bin/
     - name: vermin (Repositories)
-      run: vermin --backport argparse --violations --backport typing -t=2.6- -t=3.5- -vvv var/spack/repos
+      run: vermin --backport argparse --violations --backport typing -t=2.7- -t=3.5- -vvv var/spack/repos
   # Run style checks on the files that have been changed
   style:
     runs-on: ubuntu-latest
@@ -193,39 +193,6 @@ jobs:
       if: ${{ needs.changes.outputs.with_coverage == 'true' }}
       with:
         flags: shelltests,linux
-  # Test for Python2.6 run on Centos 6
-  centos6:
-    needs: [ validate, style, changes ]
-    runs-on: ubuntu-latest
-    container: spack/github-actions:centos6
-    steps:
-    - name: Run unit tests (full test-suite)
-      # The CentOS 6 container doesn't run with coverage, but
-      # under the same conditions it runs the full test suite
-      if: ${{ needs.changes.outputs.with_coverage == 'true' }}
-      env:
-          HOME: /home/spack-test
-          SPACK_TEST_SOLVER: original
-      run: |
-          whoami && echo $HOME && cd $HOME
-          git clone "${{ github.server_url }}/${{ github.repository }}.git" && cd spack
-          git fetch origin "${{ github.ref }}:test-branch"
-          git checkout test-branch
-          . .github/workflows/setup_git.sh
-          bin/spack unit-test -x
-    - name: Run unit tests (only package tests)
-      if: ${{ needs.changes.outputs.with_coverage == 'false' }}
-      env:
-          HOME: /home/spack-test
-          ONLY_PACKAGES: true
-          SPACK_TEST_SOLVER: original
-      run: |
-          whoami && echo $HOME && cd $HOME
-          git clone "${{ github.server_url }}/${{ github.repository }}.git" && cd spack
-          git fetch origin "${{ github.ref }}:test-branch"
-          git checkout test-branch
-          . .github/workflows/setup_git.sh
-          bin/spack unit-test -x -m "not maybeslow" -k "package_sanity"
 
   # Test RHEL8 UBI with platform Python. This job is run
   # only on PRs modifying core Spack
diff --git a/bin/spack b/bin/spack
index dffcbd2026..3b4f782d1c 100755
--- a/bin/spack
+++ b/bin/spack
@@ -33,11 +33,11 @@ import sys
 
 min_python3 = (3, 5)
 
-if sys.version_info[:2] < (2, 6) or (
+if sys.version_info[:2] < (2, 7) or (
     sys.version_info[:2] >= (3, 0) and sys.version_info[:2] < min_python3
 ):
     v_info = sys.version_info[:3]
-    msg = "Spack requires Python 2.6, 2.7 or %d.%d or higher " % min_python3
+    msg = "Spack requires Python 2.7 or %d.%d or higher " % min_python3
     msg += "You are running spack with Python %d.%d.%d." % v_info
     sys.exit(msg)
 
@@ -54,8 +54,6 @@ spack_external_libs = os.path.join(spack_lib_path, "external")
 
 if sys.version_info[:2] <= (2, 7):
     sys.path.insert(0, os.path.join(spack_external_libs, "py2"))
-if sys.version_info[:2] == (2, 6):
-    sys.path.insert(0, os.path.join(spack_external_libs, "py26"))
 
 sys.path.insert(0, spack_external_libs)
 
diff --git a/lib/spack/docs/contribution_guide.rst b/lib/spack/docs/contribution_guide.rst
index 17e24b816e..4d639a1c9b 100644
--- a/lib/spack/docs/contribution_guide.rst
+++ b/lib/spack/docs/contribution_guide.rst
@@ -71,7 +71,7 @@ locally to speed up the review process.
    new release that is causing problems. If this is the case, please file an issue.
 
 
-We currently test against Python 2.6, 2.7, and 3.5-3.7 on both macOS and Linux and
+We currently test against Python 2.7 and 3.5-3.9 on both macOS and Linux and
 perform 3 types of tests:
 
 .. _cmd-spack-unit-test:
diff --git a/lib/spack/docs/tables/system_prerequisites.csv b/lib/spack/docs/tables/system_prerequisites.csv
index 074be4bffb..980aea77f6 100644
--- a/lib/spack/docs/tables/system_prerequisites.csv
+++ b/lib/spack/docs/tables/system_prerequisites.csv
@@ -1,5 +1,5 @@
 Name, Supported Versions, Notes, Requirement Reason
-Python, 2.6/2.7/3.5-3.9, , Interpreter for Spack
+Python, 2.7/3.5-3.9, , Interpreter for Spack
 C/C++ Compilers, , , Building software
 make, , , Build software
 patch, , , Build software
diff --git a/lib/spack/external/__init__.py b/lib/spack/external/__init__.py
index be4b9fbf14..995b98146d 100644
--- a/lib/spack/external/__init__.py
+++ b/lib/spack/external/__init__.py
@@ -17,7 +17,7 @@ argparse
 --------
 
 * Homepage: https://pypi.python.org/pypi/argparse
-* Usage: We include our own version to be Python 2.6 compatible.
+* Usage: We include our own version to be Python 3.X compatible.
 * Version: 1.4.0
 * Note: This package has been slightly modified to improve
   error message formatting. See the following commit if the
@@ -39,15 +39,6 @@ distro
 * Usage: Provides a more stable linux distribution detection.
 * Version: 1.0.4 (last version supporting Python 2.6)
 
-functools
----------
-
-* Homepage: https://github.com/python/cpython/blob/2.7/Lib/functools.py
-* Usage: Used for implementation of total_ordering.
-* Version: Unversioned
-* Note: This is the functools.total_ordering implementation
-  from Python 2.7 backported so we can run on Python 2.6.
-
 jinja2
 ------
 
@@ -73,14 +64,6 @@ markupsafe
 * Usage: Implements a XML/HTML/XHTML Markup safe string for Python.
 * Version: 1.0
 
-orderddict
-----------
-
-* Homepage: https://pypi.org/project/ordereddict/
-* Usage: A drop-in substitute for Py2.7's new collections.OrderedDict
-  that works in Python 2.4-2.6.
-* Version: 1.1
-
 py
 --
 
diff --git a/lib/spack/external/functools_backport.py b/lib/spack/external/functools_backport.py
deleted file mode 100644
index b3c913ffd7..0000000000
--- a/lib/spack/external/functools_backport.py
+++ /dev/null
@@ -1,47 +0,0 @@
-#
-# Backport of Python 2.7's total_ordering.
-#
-
-def total_ordering(cls):
-    """Class decorator that fills in missing ordering methods"""
-    convert = {
-        '__lt__': [('__gt__', lambda self, other: not (self < other or self == other)),
-                   ('__le__', lambda self, other: self < other or self == other),
-                   ('__ge__', lambda self, other: not self < other)],
-        '__le__': [('__ge__', lambda self, other: not self <= other or self == other),
-                   ('__lt__', lambda self, other: self <= other and not self == other),
-                   ('__gt__', lambda self, other: not self <= other)],
-        '__gt__': [('__lt__', lambda self, other: not (self > other or self == other)),
-                   ('__ge__', lambda self, other: self > other or self == other),
-                   ('__le__', lambda self, other: not self > other)],
-        '__ge__': [('__le__', lambda self, other: (not self >= other) or self == other),
-                   ('__gt__', lambda self, other: self >= other and not self == other),
-                   ('__lt__', lambda self, other: not self >= other)]
-    }
-    roots = set(dir(cls)) & set(convert)
-    if not roots:
-        raise ValueError('must define at least one ordering operation: < > <= >=')
-    root = max(roots)       # prefer __lt__ to __le__ to __gt__ to __ge__
-    for opname, opfunc in convert[root]:
-        if opname not in roots:
-            opfunc.__name__ = opname
-            opfunc.__doc__ = getattr(int, opname).__doc__
-            setattr(cls, opname, opfunc)
-    return cls
-
-
-@total_ordering
-class reverse_order(object):
-    """Helper for creating key functions.
-
-       This is a wrapper that inverts the sense of the natural
-       comparisons on the object.
-    """
-    def __init__(self, value):
-        self.value = value
-
-    def __eq__(self, other):
-        return other.value == self.value
-
-    def __lt__(self, other):
-        return other.value < self.value
diff --git a/lib/spack/external/ordereddict_backport.py b/lib/spack/external/ordereddict_backport.py
deleted file mode 100644
index 3c7f012e9e..0000000000
--- a/lib/spack/external/ordereddict_backport.py
+++ /dev/null
@@ -1,22 +0,0 @@
-# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
-# Spack Project Developers. See the top-level COPYRIGHT file for details.
-#
-# SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-"""This file dispatches to the correct implementation of OrderedDict."""
-
-# TODO: this file, along with py26/ordereddict.py, can be removed when
-# TODO: support for python 2.6 will be dropped
-
-# Removing this import will make python 2.6
-# fail on import of ordereddict
-from __future__ import absolute_import
-
-import sys
-
-if sys.version_info[:2] == (2, 6):
-    import ordereddict
-    OrderedDict = ordereddict.OrderedDict
-else:
-    import collections
-    OrderedDict = collections.OrderedDict
diff --git a/lib/spack/external/py26/ordereddict.py b/lib/spack/external/py26/ordereddict.py
deleted file mode 100644
index 7242b5060d..0000000000
--- a/lib/spack/external/py26/ordereddict.py
+++ /dev/null
@@ -1,127 +0,0 @@
-# Copyright (c) 2009 Raymond Hettinger
-#
-# Permission is hereby granted, free of charge, to any person
-# obtaining a copy of this software and associated documentation files
-# (the "Software"), to deal in the Software without restriction,
-# including without limitation the rights to use, copy, modify, merge,
-# publish, distribute, sublicense, and/or sell copies of the Software,
-# and to permit persons to whom the Software is furnished to do so,
-# subject to the following conditions:
-#
-#     The above copyright notice and this permission notice shall be
-#     included in all copies or substantial portions of the Software.
-#
-#     THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
-#     EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
-#     OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
-#     NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
-#     HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
-#     WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
-#     FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
-#     OTHER DEALINGS IN THE SOFTWARE.
-
-from UserDict import DictMixin
-
-class OrderedDict(dict, DictMixin):
-
-    def __init__(self, *args, **kwds):
-        if len(args) > 1:
-            raise TypeError('expected at most 1 arguments, got %d' % len(args))
-        try:
-            self.__end
-        except AttributeError:
-            self.clear()
-        self.update(*args, **kwds)
-
-    def clear(self):
-        self.__end = end = []
-        end += [None, end, end]         # sentinel node for doubly linked list
-        self.__map = {}                 # key --> [key, prev, next]
-        dict.clear(self)
-
-    def __setitem__(self, key, value):
-        if key not in self:
-            end = self.__end
-            curr = end[1]
-            curr[2] = end[1] = self.__map[key] = [key, curr, end]
-        dict.__setitem__(self, key, value)
-
-    def __delitem__(self, key):
-        dict.__delitem__(self, key)
-        key, prev, next = self.__map.pop(key)
-        prev[2] = next
-        next[1] = prev
-
-    def __iter__(self):
-        end = self.__end
-        curr = end[2]
-        while curr is not end:
-            yield curr[0]
-            curr = curr[2]
-
-    def __reversed__(self):
-        end = self.__end
-        curr = end[1]
-        while curr is not end:
-            yield curr[0]
-            curr = curr[1]
-
-    def popitem(self, last=True):
-        if not self:
-            raise KeyError('dictionary is empty')
-        if last:
-            key = reversed(self).next()
-        else:
-            key = iter(self).next()
-        value = self.pop(key)
-        return key, value
-
-    def __reduce__(self):
-        items = [[k, self[k]] for k in self]
-        tmp = self.__map, self.__end
-        del self.__map, self.__end
-        inst_dict = vars(self).copy()
-        self.__map, self.__end = tmp
-        if inst_dict:
-            return (self.__class__, (items,), inst_dict)
-        return self.__class__, (items,)
-
-    def keys(self):
-        return list(self)
-
-    setdefault = DictMixin.setdefault
-    update = DictMixin.update
-    pop = DictMixin.pop
-    values = DictMixin.values
-    items = DictMixin.items
-    iterkeys = DictMixin.iterkeys
-    itervalues = DictMixin.itervalues
-    iteritems = DictMixin.iteritems
-
-    def __repr__(self):
-        if not self:
-            return '%s()' % (self.__class__.__name__,)
-        return '%s(%r)' % (self.__class__.__name__, self.items())
-
-    def copy(self):
-        return self.__class__(self)
-
-    @classmethod
-    def fromkeys(cls, iterable, value=None):
-        d = cls()
-        for key in iterable:
-            d[key] = value
-        return d
-
-    def __eq__(self, other):
-        if isinstance(other, OrderedDict):
-            if len(self) != len(other):
-                return False
-            for p, q in  zip(self.items(), other.items()):
-                if p != q:
-                    return False
-            return True
-        return dict.__eq__(self, other)
-
-    def __ne__(self, other):
-        return not self == other
diff --git a/lib/spack/external/pytest-fallback/_pytest/fixtures.py b/lib/spack/external/pytest-fallback/_pytest/fixtures.py
index 98317a4889..7ad495615e 100644
--- a/lib/spack/external/pytest-fallback/_pytest/fixtures.py
+++ b/lib/spack/external/pytest-fallback/_pytest/fixtures.py
@@ -1,5 +1,6 @@
 from __future__ import absolute_import, division, print_function
 
+import collections
 import inspect
 import sys
 import warnings
@@ -21,9 +22,6 @@ from _pytest.compat import (
 from _pytest.outcomes import fail, TEST_OUTCOME
 
 
-from ordereddict_backport import OrderedDict
-
-
 def pytest_sessionstart(session):
     import _pytest.python
     scopename2class.update({
@@ -165,7 +163,7 @@ def reorder_items(items):
     for scopenum in range(0, scopenum_function):
         argkeys_cache[scopenum] = d = {}
         for item in items:
-            keys = OrderedDict.fromkeys(get_parametrized_fixture_keys(item, scopenum))
+            keys = collections.OrderedDict.fromkeys(get_parametrized_fixture_keys(item, scopenum))
             if keys:
                 d[item] = keys
     return reorder_items_atscope(items, set(), argkeys_cache, 0)
@@ -200,7 +198,7 @@ def slice_items(items, ignore, scoped_argkeys_cache):
         for i, item in enumerate(it):
             argkeys = scoped_argkeys_cache.get(item)
             if argkeys is not None:
-                newargkeys = OrderedDict.fromkeys(k for k in argkeys if k not in ignore)
+                newargkeys = collections.OrderedDict.fromkeys(k for k in argkeys if k not in ignore)
                 if newargkeys:  # found a slicing key
                     slicing_argkey, _ = newargkeys.popitem()
                     items_before = items[:i]
diff --git a/lib/spack/llnl/util/multiproc.py b/lib/spack/llnl/util/multiproc.py
index c73ebaed20..2a261d143e 100644
--- a/lib/spack/llnl/util/multiproc.py
+++ b/lib/spack/llnl/util/multiproc.py
@@ -16,7 +16,7 @@ __all__ = ['Barrier']
 class Barrier:
     """Simple reusable semaphore barrier.
 
-    Python 2.6 doesn't have multiprocessing barriers so we implement this.
+    Python 2 doesn't have multiprocessing barriers so we implement this.
 
     See http://greenteapress.com/semaphores/downey08semaphores.pdf, p. 41.
     """
diff --git a/lib/spack/spack/binary_distribution.py b/lib/spack/spack/binary_distribution.py
index 0a23896b8f..99566a23ae 100644
--- a/lib/spack/spack/binary_distribution.py
+++ b/lib/spack/spack/binary_distribution.py
@@ -4,6 +4,7 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 import codecs
+import collections
 import hashlib
 import json
 import os
@@ -15,7 +16,6 @@ import traceback
 from contextlib import closing
 
 import ruamel.yaml as yaml
-from ordereddict_backport import OrderedDict
 from six.moves.urllib.error import HTTPError, URLError
 
 import llnl.util.lang
@@ -1278,8 +1278,8 @@ def relocate_package(spec, allow_root):
     # Spurious replacements (e.g. sbang) will cause issues with binaries
     # For example, the new sbang can be longer than the old one.
     # Hence 2 dictionaries are maintained here.
-    prefix_to_prefix_text = OrderedDict({})
-    prefix_to_prefix_bin = OrderedDict({})
+    prefix_to_prefix_text = collections.OrderedDict()
+    prefix_to_prefix_bin = collections.OrderedDict()
 
     if old_sbang_install_path:
         install_path = spack.hooks.sbang.sbang_install_path()
diff --git a/lib/spack/spack/bootstrap.py b/lib/spack/spack/bootstrap.py
index f7e555bea8..f046a34a19 100644
--- a/lib/spack/spack/bootstrap.py
+++ b/lib/spack/spack/bootstrap.py
@@ -12,15 +12,10 @@ import os
 import os.path
 import re
 import sys
+import sysconfig
 
 import six
 
-try:
-    import sysconfig  # novm
-except ImportError:
-    # Not supported on Python 2.6
-    pass
-
 import archspec.cpu
 
 import llnl.util.filesystem as fs
diff --git a/lib/spack/spack/cmd/license.py b/lib/spack/spack/cmd/license.py
index 3627b35391..82cbc3b2a7 100644
--- a/lib/spack/spack/cmd/license.py
+++ b/lib/spack/spack/cmd/license.py
@@ -46,9 +46,8 @@ licensed_files = [
     r'^lib/spack/docs/.*\.py$',
     r'^lib/spack/docs/spack.yaml$',
 
-    # 2 files in external
+    # 1 file in external
     r'^lib/spack/external/__init__.py$',
-    r'^lib/spack/external/ordereddict_backport.py$',
 
     # shell scripts in share
     r'^share/spack/.*\.sh$',
diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index f3572cfdcf..90b4c5b669 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -16,14 +16,13 @@ TODO: make this customizable and allow users to configure
 """
 from __future__ import print_function
 
+import functools
 import os.path
 import platform
 import tempfile
 from contextlib import contextmanager
 from itertools import chain
 
-from functools_backport import reverse_order
-
 import archspec.cpu
 
 import llnl.util.filesystem as fs
@@ -48,6 +47,23 @@ from spack.version import Version, VersionList, VersionRange, ver
 _abi = llnl.util.lang.Singleton(lambda: spack.abi.ABI())
 
 
+@functools.total_ordering
+class reverse_order(object):
+    """Helper for creating key functions.
+
+       This is a wrapper that inverts the sense of the natural
+       comparisons on the object.
+    """
+    def __init__(self, value):
+        self.value = value
+
+    def __eq__(self, other):
+        return other.value == self.value
+
+    def __lt__(self, other):
+        return other.value < self.value
+
+
 class Concretizer(object):
     """You can subclass this class to override some of the default
        concretization strategies, or you can override all of them.
@@ -130,11 +146,11 @@ class Concretizer(object):
 
         # Use a sort key to order the results
         return sorted(usable, key=lambda spec: (
-            not spec.external,                            # prefer externals
-            pref_key(spec),                               # respect prefs
-            spec.name,                                    # group by name
-            reverse_order(spec.versions),                 # latest version
-            spec                                          # natural order
+            not spec.external,             # prefer externals
+            pref_key(spec),                # respect prefs
+            spec.name,                     # group by name
+            reverse_order(spec.versions),  # latest version
+            spec                           # natural order
         ))
 
     def choose_virtual_or_external(self, spec):
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index f3fc73e4b4..d967fef122 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -39,7 +39,6 @@ from contextlib import contextmanager
 from typing import List  # novm
 
 import ruamel.yaml as yaml
-from ordereddict_backport import OrderedDict
 from ruamel.yaml.error import MarkedYAMLError
 from six import iteritems
 
@@ -401,7 +400,7 @@ class Configuration(object):
                 Configuration, ordered from lowest to highest precedence
 
         """
-        self.scopes = OrderedDict()
+        self.scopes = collections.OrderedDict()
         for scope in scopes:
             self.push_scope(scope)
         self.format_updates = collections.defaultdict(list)
diff --git a/lib/spack/spack/environment/environment.py b/lib/spack/spack/environment/environment.py
index a6be5d1c50..e876d014dd 100644
--- a/lib/spack/spack/environment/environment.py
+++ b/lib/spack/spack/environment/environment.py
@@ -13,7 +13,6 @@ import time
 
 import ruamel.yaml as yaml
 import six
-from ordereddict_backport import OrderedDict
 
 import llnl.util.filesystem as fs
 import llnl.util.tty as tty
@@ -318,7 +317,11 @@ class ViewDescriptor(object):
             # projections guaranteed to be ordered dict if true-ish
             # for python2.6, may be syaml or ruamel.yaml implementation
             # so we have to check for both
-            types = (OrderedDict, syaml.syaml_dict, yaml.comments.CommentedMap)
+            types = (
+                collections.OrderedDict,
+                syaml.syaml_dict,
+                yaml.comments.CommentedMap
+            )
             assert isinstance(self.projections, types)
             ret['projections'] = self.projections
         if self.select:
@@ -638,7 +641,7 @@ class Environment(object):
         else:
             self.raw_yaml, self.yaml = _read_yaml(f)
 
-        self.spec_lists = OrderedDict()
+        self.spec_lists = collections.OrderedDict()
 
         for item in config_dict(self.yaml).get('definitions', []):
             entry = copy.deepcopy(item)
diff --git a/lib/spack/spack/extensions.py b/lib/spack/spack/extensions.py
index 69a287fe85..0a1a056819 100644
--- a/lib/spack/spack/extensions.py
+++ b/lib/spack/spack/extensions.py
@@ -5,6 +5,7 @@
 """Service functions and classes to implement the hooks
 for Spack's command extensions.
 """
+import importlib
 import os
 import re
 import sys
@@ -98,9 +99,7 @@ def load_command_extension(command, path):
     ensure_package_creation(extension)
     ensure_package_creation(extension + '.cmd')
 
-    # TODO: Upon removal of support for Python 2.6 substitute the call
-    # TODO: below with importlib.import_module(module_name)
-    module = llnl.util.lang.load_module_from_file(module_name, cmd_path)
+    module = importlib.import_module(module_name)
     sys.modules[module_name] = module
 
     return module
diff --git a/lib/spack/spack/filesystem_view.py b/lib/spack/spack/filesystem_view.py
index 659ae550ae..741e38490a 100644
--- a/lib/spack/spack/filesystem_view.py
+++ b/lib/spack/spack/filesystem_view.py
@@ -3,14 +3,13 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
+import collections
 import functools as ft
 import os
 import re
 import shutil
 import sys
 
-from ordereddict_backport import OrderedDict
-
 from llnl.util import tty
 from llnl.util.filesystem import mkdirp, remove_dead_links, remove_empty_directories
 from llnl.util.lang import index_by, match_predicate
@@ -79,7 +78,7 @@ def view_copy(src, dst, view, spec=None):
         orig_sbang = '#!/bin/bash {0}/bin/sbang'.format(spack.paths.spack_root)
         new_sbang = sbang.sbang_shebang_line()
 
-        prefix_to_projection = OrderedDict({
+        prefix_to_projection = collections.OrderedDict({
             spec.prefix: view.get_projection_for_spec(spec)})
 
         for dep in spec.traverse():
diff --git a/lib/spack/spack/mirror.py b/lib/spack/spack/mirror.py
index 2d1a3e819e..0d51c26ce2 100644
--- a/lib/spack/spack/mirror.py
+++ b/lib/spack/spack/mirror.py
@@ -11,6 +11,7 @@ the main server for a particular package is down.  Or, if the computer
 where spack is run is not connected to the internet, it allows spack
 to download packages directly from a mirror (e.g., on an intranet).
 """
+import collections
 import operator
 import os
 import os.path
@@ -19,7 +20,6 @@ import traceback
 
 import ruamel.yaml.error as yaml_error
 import six
-from ordereddict_backport import OrderedDict
 
 if sys.version_info >= (3, 5):
     from collections.abc import Mapping  # novm
@@ -230,7 +230,7 @@ class MirrorCollection(Mapping):
     """A mapping of mirror names to mirrors."""
 
     def __init__(self, mirrors=None, scope=None):
-        self._mirrors = OrderedDict(
+        self._mirrors = collections.OrderedDict(
             (name, Mirror.from_dict(mirror, name))
             for name, mirror in (
                 mirrors.items() if mirrors is not None else
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index 8e1e81c124..77a881c442 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -29,7 +29,6 @@ import types
 from typing import Any, Callable, Dict, List, Optional  # novm
 
 import six
-from ordereddict_backport import OrderedDict
 
 import llnl.util.filesystem as fsys
 import llnl.util.tty as tty
@@ -902,7 +901,7 @@ class PackageBase(six.with_metaclass(PackageMeta, PackageViewMixin, object)):
         explicitly defined ``url`` argument. So, this list may be empty
         if a package only defines ``url`` at the top level.
         """
-        version_urls = OrderedDict()
+        version_urls = collections.OrderedDict()
         for v, args in sorted(self.versions.items()):
             if 'url' in args:
                 version_urls[v] = args['url']
diff --git a/lib/spack/spack/relocate.py b/lib/spack/spack/relocate.py
index 14bc0e1953..08ec1a18ee 100644
--- a/lib/spack/spack/relocate.py
+++ b/lib/spack/spack/relocate.py
@@ -2,15 +2,14 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
+import collections
 import multiprocessing.pool
 import os
 import re
 import shutil
-from collections import defaultdict
 
 import macholib.mach_o
 import macholib.MachO
-from ordereddict_backport import OrderedDict
 
 import llnl.util.lang
 import llnl.util.tty as tty
@@ -807,7 +806,7 @@ def relocate_text(files, prefixes, concurrency=32):
     # orig_sbang = '#!/bin/bash {0}/bin/sbang'.format(orig_spack)
     # new_sbang = '#!/bin/bash {0}/bin/sbang'.format(new_spack)
 
-    compiled_prefixes = OrderedDict({})
+    compiled_prefixes = collections.OrderedDict({})
 
     for orig_prefix, new_prefix in prefixes.items():
         if orig_prefix != new_prefix:
@@ -845,7 +844,7 @@ def relocate_text_bin(binaries, prefixes, concurrency=32):
     Raises:
       BinaryTextReplaceError: when the new path is longer than the old path
     """
-    byte_prefixes = OrderedDict({})
+    byte_prefixes = collections.OrderedDict({})
 
     for orig_prefix, new_prefix in prefixes.items():
         if orig_prefix != new_prefix:
@@ -1032,7 +1031,7 @@ def fixup_macos_rpath(root, filename):
     # Convert rpaths list to (name -> number of occurrences)
     add_rpaths = set()
     del_rpaths = set()
-    rpaths = defaultdict(int)
+    rpaths = collections.defaultdict(int)
     for rpath in rpath_list:
         rpaths[rpath] += 1
 
diff --git a/lib/spack/spack/reporters/cdash.py b/lib/spack/spack/reporters/cdash.py
index 19721fc676..73c75a1e18 100644
--- a/lib/spack/spack/reporters/cdash.py
+++ b/lib/spack/spack/reporters/cdash.py
@@ -2,9 +2,8 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-
 import codecs
+import collections
 import hashlib
 import os.path
 import platform
@@ -13,7 +12,6 @@ import socket
 import time
 import xml.sax.saxutils
 
-from ordereddict_backport import OrderedDict
 from six import iteritems, text_type
 from six.moves.urllib.parse import urlencode
 from six.moves.urllib.request import HTTPHandler, Request, build_opener
@@ -96,7 +94,7 @@ class CDash(Reporter):
             buildstamp_format = "%Y%m%d-%H%M-{0}".format(args.cdash_track)
             self.buildstamp = time.strftime(buildstamp_format,
                                             time.localtime(self.endtime))
-        self.buildIds = OrderedDict()
+        self.buildIds = collections.OrderedDict()
         self.revision = ''
         git = which('git')
         with working_dir(spack.paths.spack_root):
diff --git a/lib/spack/spack/test/cmd/url.py b/lib/spack/spack/test/cmd/url.py
index f9179720b1..249ea6ec57 100644
--- a/lib/spack/spack/test/cmd/url.py
+++ b/lib/spack/spack/test/cmd/url.py
@@ -3,7 +3,6 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 import re
-import sys
 
 import pytest
 
@@ -69,14 +68,6 @@ def test_url_with_no_version_fails():
         url('parse', 'http://www.netlib.org/voronoi/triangle.zip')
 
 
-skip_python_26 = pytest.mark.skipif(
-    sys.version_info < (2, 7),
-    reason="Python 2.6 tests are run in a container, where "
-           "networking is super slow"
-)
-
-
-@skip_python_26
 def test_url_list(mock_packages):
     out = url('list')
     total_urls = len(out.split('\n'))
@@ -106,7 +97,6 @@ def test_url_list(mock_packages):
     assert 0 < correct_version_urls < total_urls
 
 
-@skip_python_26
 def test_url_summary(mock_packages):
     """Test the URL summary command."""
     # test url_summary, the internal function that does the work
@@ -133,7 +123,6 @@ def test_url_summary(mock_packages):
     assert out_correct_versions == correct_versions
 
 
-@skip_python_26
 def test_url_stats(capfd, mock_packages):
     with capfd.disabled():
         output = url('stats')
diff --git a/lib/spack/spack/test/llnl/util/tty/log.py b/lib/spack/spack/test/llnl/util/tty/log.py
index 78d4e24fa4..983dea9c0b 100644
--- a/lib/spack/spack/test/llnl/util/tty/log.py
+++ b/lib/spack/spack/test/llnl/util/tty/log.py
@@ -429,10 +429,6 @@ def mock_shell_v_v_no_termios(proc, ctl, **kwargs):
     (mock_shell_v_v, nullcontext),
     (mock_shell_v_v_no_termios, no_termios),
 ])
-@pytest.mark.skipif(
-    sys.version_info < (2, 7),
-    reason="Python 2.6 tests are run in a container, where this fails often"
-)
 def test_foreground_background_output(
         test_fn, capfd, termios_on_or_off, tmpdir):
     """Tests hitting 'v' toggles output, and that force_echo works."""
diff --git a/lib/spack/spack/test/web.py b/lib/spack/spack/test/web.py
index b940fe6055..4ac0da162b 100644
--- a/lib/spack/spack/test/web.py
+++ b/lib/spack/spack/test/web.py
@@ -2,9 +2,9 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
+import collections
 import os
 
-import ordereddict_backport
 import pytest
 
 import llnl.util.tty as tty
@@ -152,7 +152,7 @@ def test_get_header():
     # If lookup has to fallback to fuzzy matching and there are more than one
     # fuzzy match, the result depends on the internal ordering of the given
     # mapping
-    headers = ordereddict_backport.OrderedDict()
+    headers = collections.OrderedDict()
     headers['Content-type'] = 'text/plain'
     headers['contentType'] = 'text/html'
 
@@ -161,7 +161,7 @@ def test_get_header():
     assert(spack.util.web.get_header(headers, 'CONTENT_TYPE') == 'text/html')
 
     # Same as above, but different ordering
-    headers = ordereddict_backport.OrderedDict()
+    headers = collections.OrderedDict()
     headers['contentType'] = 'text/html'
     headers['Content-type'] = 'text/plain'
 
diff --git a/lib/spack/spack/util/mock_package.py b/lib/spack/spack/util/mock_package.py
index d8b3ec468b..ab7b8439e5 100644
--- a/lib/spack/spack/util/mock_package.py
+++ b/lib/spack/spack/util/mock_package.py
@@ -2,10 +2,8 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
 """Infrastructure used by tests for mocking packages and repos."""
-
-import ordereddict_backport
+import collections
 
 import spack.provider_index
 import spack.util.naming
@@ -149,7 +147,7 @@ class MockPackageMultiRepo(object):
         MockPackage._repo = self
 
         # set up dependencies
-        MockPackage.dependencies = ordereddict_backport.OrderedDict()
+        MockPackage.dependencies = collections.OrderedDict()
         for dep, dtype in zip(dependencies, dependency_types):
             d = Dependency(MockPackage, Spec(dep.name), type=dtype)
             if not conditions or dep.name not in conditions:
diff --git a/lib/spack/spack/util/spack_yaml.py b/lib/spack/spack/util/spack_yaml.py
index a67b6491f8..9812943a79 100644
--- a/lib/spack/spack/util/spack_yaml.py
+++ b/lib/spack/spack/util/spack_yaml.py
@@ -12,13 +12,13 @@
   default unorderd dict.
 
 """
+import collections
 import ctypes
 import re
 import sys
 from typing import List  # novm
 
 import ruamel.yaml as yaml
-from ordereddict_backport import OrderedDict
 from ruamel.yaml import RoundTripDumper, RoundTripLoader
 from six import StringIO, string_types
 
@@ -39,7 +39,7 @@ __all__ = ['load', 'dump', 'SpackYAMLError']
 # Also, use OrderedDict instead of just dict.
 
 
-class syaml_dict(OrderedDict):
+class syaml_dict(collections.OrderedDict):
     def __repr__(self):
         mappings = ('%r: %r' % (k, v) for k, v in self.items())
         return '{%s}' % ', '.join(mappings)