⚗️ upload to Test PyPI when running CD workflow

[burgholzer]

Description

This PR updates the CD workflow so that anytime it runs, it uploads the resulting packages to Test PyPI. This allows to battle test the packages before official releases. This will be especially important in the context of #662.

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.

[burgholzer]

Unfortunately, it seems that this is not really feasible. Test PyPI won't accept packages with local version identifiers and setuptools-scm does not easily allow to disable local versions, e.g., based on the presence of an environment variable. Maybe sometime in the future.