⚗️ upload to Test PyPI for pushes on main

[burgholzer]

Description

This PR updates the CD workflow so that anytime it runs on a push to main, it uploads the resulting package to Test PyPI. This allows to battle test the packages before official releases.
This will be especially important in the context of #432

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.4%. Comparing base (fec5499) to head (0bd41c5).
Report is 8 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #461   +/-   ##
=====================================
  Coverage   95.4%   95.4%           
=====================================
  Files         40      40           
  Lines       1834    1835    +1     
  Branches     223     224    +1     
=====================================
+ Hits        1751    1752    +1     
  Misses        83      83           

Flag	Coverage Δ
cpp	95.9% <ø> (+<0.1%)	⬆️
python	93.7% <ø> (ø)

see 1 file with indirect coverage changes