Add mypy config for local tests

[cdce8p]

Astroid isn't fully typed yet and thus doesn't have a py.typed file yet. Sometimes it's helpful to check the mypy output as if astroid would have a py.typed file. Mypy does have an option for it follow_untyped_imports = true. This PR adds it to the config as a comment. If necessary devs can simply uncomment that section and run mypy without having to lookup the correct config value again.

https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-library-stubs-or-py-typed-marker
https://mypy.readthedocs.io/en/stable/config_file.html#confval-follow_untyped_imports

Unfortunately there are still over 900 errors with that setting enabled, so I don't think we'll be adding py.typed to astroid any time soon.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.94%. Comparing base (6f91463) to head (16ee5ad).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10635   +/-   ##
=======================================
  Coverage   95.94%   95.94%           
=======================================
  Files         176      176           
  Lines       19519    19519           
=======================================
  Hits        18727    18727           
  Misses        792      792           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.