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

5 files changed, 4 insertions, 219 deletions

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]