chore(ruff): enable flake8-no-pep420

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

Enable https://docs.astral.sh/ruff/rules/implicit-namespace-package/, as this is known to cause some issues with some tools. The original repository provides a bit more information about the rationale.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.4%. Comparing base (af87217) to head (c41b741).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #659   +/-   ##
=====================================
  Coverage   92.4%   92.4%           
=====================================
  Files         35      35           
  Lines        950     950           
  Branches     194     194           
=====================================
  Hits         878     878           
  Misses        55      55           
  Partials      17      17           

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