`spack style`: automatically bootstrap dependencies

This uses our bootstrapping logic to automatically install dependencies for
`spack style`. Users should no longer have to pre-install all of the tools
(`isort`, `mypy`, `black`, `flake8`). The command will do it for them.

- [x] add logic to bootstrap specs with specific version requirements in `spack style`
- [x] remove style tools from CI requirements (to ensure we test bootstrapping)
- [x] rework dependencies for `mypy` and `py-typed-ast`
      - `py-typed-ast` needs to be a link dependency
      - it needs to be at 1.4.1 or higher to work with python 3.9

Signed-off-by: vsoch <vsoch@users.noreply.github.com>

2 files changed, 43 insertions, 8 deletions

diff --git a/lib/spack/docs/developer_guide.rst b/lib/spack/docs/developer_guide.rst
index 31311e87f7..71aac1fd93 100644
--- a/lib/spack/docs/developer_guide.rst
+++ b/lib/spack/docs/developer_guide.rst
@@ -818,6 +818,29 @@ Developer commands
 ``spack doc``
 ^^^^^^^^^^^^^
 
+.. _cmd-spack-style:
+
+^^^^^^^^^^^^^^^
+``spack style``
+^^^^^^^^^^^^^^^
+
+spack style exists to help the developer user to check imports and style with
+mypy, flake8, isort, and (soon) black. To run all style checks, simply do:
+
+.. code-block:: console
+
+    $ spack style
+
+To run automatic fixes for isort you can do:
+
+.. code-block:: console
+
+    $ spack style --fix
+
+You do not need any of these Python packages installed on your system for
+the checks to work! Spack will bootstrap install them from packages for
+your use.
+
 ^^^^^^^^^^^^^^^^^^^
 ``spack unit-test``
 ^^^^^^^^^^^^^^^^^^^
diff --git a/lib/spack/spack/cmd/style.py b/lib/spack/spack/cmd/style.py
index 0eceb4dc2d..2841a4af0f 100644
--- a/lib/spack/spack/cmd/style.py
+++ b/lib/spack/spack/cmd/style.py
@@ -14,6 +14,7 @@ import llnl.util.tty as tty
 import llnl.util.tty.color as color
 from llnl.util.filesystem import working_dir
 
+import spack.bootstrap
 import spack.paths
 from spack.util.executable import which
 
@@ -44,9 +45,15 @@ initial_working_dir = None
 #: List of directories to exclude from checks.
 exclude_directories = [spack.paths.external_path]
 
-#: order in which tools should be run. flake8 is last so that it can
+#: Order in which tools should be run. flake8 is last so that it can
 #: double-check the results of other tools (if, e.g., --fix was provided)
-tool_order = ["isort", "mypy", "black", "flake8"]
+#: The list maps an executable name to a spack spec needed to install it.
+tool_order = [
+    ("isort", "py-isort@4.3.5:"),
+    ("mypy", "py-mypy@0.900:"),
+    ("black", "py-black"),
+    ("flake8", "py-flake8"),
+]
 
 #: tools we run in spack style
 tools = {}
@@ -210,7 +217,7 @@ def rewrite_and_print_output(
 
 
 def print_style_header(file_list, args):
-    tools = [tool for tool in tool_order if getattr(args, tool)]
+    tools = [tool for tool, _ in tool_order if getattr(args, tool)]
     tty.msg("Running style checks on spack:", "selected: " + ", ".join(tools))
 
     # translate modified paths to cwd_relative if needed
@@ -338,15 +345,20 @@ def style(parser, args):
 
         # run tools in order defined in tool_order
         returncode = 0
-        for tool_name in tool_order:
+        for tool_name, tool_spec in tool_order:
             if getattr(args, tool_name):
                 run_function, required = tools[tool_name]
                 print_tool_header(tool_name)
 
-                cmd = which(tool_name, required=required)
-                if not cmd:
-                    color.cprint("  @y{%s not in PATH, skipped}" % tool_name)
-                    continue
+                # Bootstrap tools so we don't need to require install
+                with spack.bootstrap.ensure_bootstrap_configuration():
+                    spec = spack.spec.Spec(tool_spec)
+                    cmd = spack.bootstrap.get_executable(
+                        tool_name, spec=spec, install=True
+                    )
+                    if not cmd:
+                        color.cprint("  @y{%s not in PATH, skipped}" % tool_name)
+                        continue
 
                 returncode |= run_function(cmd, file_list, args)