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>

share/spack/qa/run-style-tests

@@ -15,7 +15,6 @@
 #     run-flake8-tests
 #
 . "$(dirname $0)/setup.sh"
-check_dependencies flake8 mypy
 
 # verify that the code style is correct
 spack style --root-relative