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>

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``
 ^^^^^^^^^^^^^^^^^^^