add mypy to style checks; rename spack flake8 to spack style (#20384)

I lost my mind a bit after getting the completion stuff working and
decided to get Mypy working for spack as well. This adds a 
`.mypy.ini` that checks all of the spack and llnl modules, though
not yet packages, and fixes all of the identified missing types and
type issues for the spack library.

In addition to these changes, this includes:

* rename `spack flake8` to `spack style`

Aliases flake8 to style, and just runs flake8 as before, but with
a warning.  The style command runs both `flake8` and `mypy`,
in sequence. Added --no-<tool> options to turn off one or the
other, they are on by default.  Fixed two issues caught by the tools.

* stub typing module for python2.x

We don't support typing in Spack for python 2.x. To allow 2.x to
support `import typing` and `from typing import ...` without a
try/except dance to support old versions, this adds a stub module
*just* for python 2.x.  Doing it this way means we can only reliably
use all type hints in python3.7+, and mypi.ini has been updated to
reflect that.

* add non-default black check to spack style

This is a first step to requiring black.  It doesn't enforce it by
default, but it will check it if requested.  Currently enforcing the
line length of 79 since that's what flake8 requires, but it's a bit odd
for a black formatted project to be quite that narrow.  All settings are
in the style command since spack has no pyproject.toml and I don't
want to add one until more discussion happens. Also re-format
`style.py` since it no longer passed the black style check
with the new length.

* use style check in github action

Update the style and docs action to use `spack style`, adding in mypy
and black to the action even if it isn't running black right now.

lib/spack/docs/contribution_guide.rst

@@ -179,24 +179,26 @@ how to write tests!
    run the unit tests yourself, we suggest you use ``spack unit-test``.
 
 ^^^^^^^^^^^^
-Flake8 Tests
+Style Tests
 ^^^^^^^^^^^^
 
 Spack uses `Flake8 <http://flake8.pycqa.org/en/latest/>`_ to test for
-`PEP 8 <https://www.python.org/dev/peps/pep-0008/>`_ conformance. PEP 8 is
+`PEP 8 <https://www.python.org/dev/peps/pep-0008/>`_ conformance and
+`mypy <https://mypy.readthedocs.io/en/stable/>` for type checking. PEP 8 is
 a series of style guides for Python that provide suggestions for everything
 from variable naming to indentation. In order to limit the number of PRs that
 were mostly style changes, we decided to enforce PEP 8 conformance. Your PR
-needs to comply with PEP 8 in order to be accepted.
+needs to comply with PEP 8 in order to be accepted, and if it modifies the
+spack library it needs to successfully type-check with mypy as well.
 
-Testing for PEP 8 compliance is easy. Simply run the ``spack flake8``
+Testing for compliance with spack's style is easy. Simply run the ``spack style``
 command:
 
 .. code-block:: console
 
-   $ spack flake8
+   $ spack style
 
-``spack flake8`` has a couple advantages over running ``flake8`` by hand:
+``spack style`` has a couple advantages over running the tools by hand:
 
 #. It only tests files that you have modified since branching off of
    ``develop``.
@@ -207,7 +209,9 @@ command:
    checks. For example, URLs are often longer than 80 characters, so we
    exempt them from line length checks. We also exempt lines that start
    with "homepage", "url", "version", "variant", "depends_on", and
-   "extends" in ``package.py`` files.
+   "extends" in ``package.py`` files.  This is now also possible when directly
+   running flake8 if you can use the ``spack`` formatter plugin included with
+   spack.
 
 More approved flake8 exemptions can be found
 `here <https://github.com/spack/spack/blob/develop/.flake8>`_.
@@ -240,13 +244,13 @@ However, if you aren't compliant with PEP 8, flake8 will complain:
 
 Most of the error messages are straightforward, but if you don't understand what
 they mean, just ask questions about them when you submit your PR. The line numbers
-will change if you add or delete lines, so simply run ``spack flake8`` again
+will change if you add or delete lines, so simply run ``spack style`` again
 to update them.
 
 .. tip::
 
    Try fixing flake8 errors in reverse order. This eliminates the need for
-   multiple runs of ``spack flake8`` just to re-compute line numbers and
+   multiple runs of ``spack style`` just to re-compute line numbers and
    makes it much easier to fix errors directly off of the CI output.
 
 .. warning::