Upgrade GitHub Action for setup-python from v5 to v6

[tleonhardt]

Upgrade GitHub Action for setup-python from v5 to v6

I learned about this from Dependabot via PR #1505 , but tests failed there due to inability to upload code coverage data to codecov

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (d0b21b5) to head (b6b0d7b).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1506      +/-   ##
==========================================
- Coverage   98.81%   98.79%   -0.03%     
==========================================
  Files          23       23              
  Lines        4961     4961              
==========================================
- Hits         4902     4901       -1     
- Misses         59       60       +1     

Flag	Coverage Δ
unittests	98.79% <ø> (-0.03%)	⬇️

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