Remove colorless theme monkeypatchfor argparse bug in 3.15.0b1

[tleonhardt]

Python 3.15.0b2 is out and available now in both uv and GitHub Actions.

This removes our monkeypatch around the argparse bug that was present in Python 3.15.0b1.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.50%. Comparing base (31d0461) to head (aad61ac).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1674      +/-   ##
==========================================
- Coverage   99.50%   99.50%   -0.01%     
==========================================
  Files          23       23              
  Lines        5689     5682       -7     
==========================================
- Hits         5661     5654       -7     
  Misses         28       28              

Flag	Coverage Δ
unittests	99.50% <ø> (-0.01%)	⬇️

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