[build] Finally to py 3.9

[felixdittrich92]

This PR:

• move finally to python 3.9

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.38%. Comparing base (a26bea5) to head (df1e838).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1647      +/-   ##
==========================================
- Coverage   96.42%   96.38%   -0.04%     
==========================================
  Files         164      164              
  Lines        7773     7773              
==========================================
- Hits         7495     7492       -3     
- Misses        278      281       +3     

Flag	Coverage Δ
unittests	96.38% <ø> (-0.04%)	⬇️

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