Move mypy configuration to pyproject.toml from setup.cfg

[tleonhardt]

Move mypy configuration to pyproject.toml from setup.cfg

Also:

• Simplify the GitHub mypy workflow to directly call mypy instead of using nox
• Add pyproject.toml file for the ext_test plugin
• Update ext_test plugin invoke stuff to use ruff instead of flake8

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.01%. Comparing base (ff46021) to head (3df5bd6).
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1344      +/-   ##
==========================================
+ Coverage   97.72%   98.01%   +0.29%     
==========================================
  Files          22       22              
  Lines        5753     5792      +39     
==========================================
+ Hits         5622     5677      +55     
+ Misses        131      115      -16     

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