⚗️ 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:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

[](https://app.codecov.io/gh/cda-tum/mqt-qcec/pull/461?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cda-tum) ```diff @@ 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](https://app.codecov.io/gh/cda-tum/mqt-qcec/pull/461/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cda-tum) | Coverage Δ | | |---|---|---| | [cpp](https://app.codecov.io/gh/cda-tum/mqt-qcec/pull/461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cda-tum) | `95.9% <ø> (+<0.1%)` | :arrow_up: | | [python](https://app.codecov.io/gh/cda-tum/mqt-qcec/pull/461/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cda-tum) | `93.7% <ø> (ø)` | | [see 1 file with indirect coverage changes](https://app.codecov.io/gh/cda-tum/mqt-qcec/pull/461/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cda-tum)