Clean up config files

[blink1073]

• Use flake8-pyproject to put flake8 config in pyproject.toml
• Rename readthedocs config file to recommended name

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.60%. Comparing base (712c453) to head (0b765d2).
Report is 323 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1031   +/-   ##
=======================================
  Coverage   72.60%   72.60%           
=======================================
  Files          64       64           
  Lines        8257     8257           
  Branches     1259     1259           
=======================================
  Hits         5995     5995           
  Misses       1847     1847           
  Partials      415      415           

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