⚗️ 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 92.3%. Comparing base (43ee4cc) to head (43ecb15).
Report is 57 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #441   +/-   ##
=====================================
  Coverage   92.3%   92.3%           
=====================================
  Files         37      37           
  Lines       2732    2732           
  Branches     310     310           
=====================================
  Hits        2523    2523           
  Misses       209     209           

Flag	Coverage Δ
cpp	94.1% <ø> (ø)
python	87.4% <ø> (ø)

see 1 file with indirect coverage changes