⚗️ 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.

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 90.4%. Comparing base (a3840dd) to head (b337dc1).
Report is 177 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #528   +/-   ##
=====================================
  Coverage   90.4%   90.4%           
=====================================
  Files         87      87           
  Lines      10052   10052           
  Branches    1722    1722           
=====================================
+ Hits        9092    9093    +1     
+ Misses       960     959    -1     

Flag	Coverage Δ
cpp	90.1% <ø> (+<0.1%)	⬆️
python	95.9% <ø> (ø)

see 1 file with indirect coverage changes