Define test suite configuration for best-practices and repos

[tim-schilling]

• Python and Django version matrixes
• GitHub actions integration
• Database configurations
• Running test suite locally
• Speed optimizations with uv and caching

I think the goal here is for clarity and long-term maintenance. We should try to stick to known technologies or popular things. If something is newer, it should be a minor difference compared to the alternatives (uv compared to poetry).

It feels like the decisions are around:

• Whether or not to use tox, nox or something else
• Which dependency manager for the github action to use

[tim-schilling]

@ontowhee Please feel free to adjust this issue how you'd like or make it your own.