F pipeline python versions

Summary

For #124 (closed). Adds unittest support for all supported Python versions.

Focus

Contrary to our initial plan, I decided to abandon pyenv since it is a pain in the ass to be run within a debian-based pipeline image and rather use the official Python 3.X docker images. These also have their problems with tox (or rather, pip within tox, see, e.g. https://gitlab.indiscale.com/caosdb/src/caosdb-pylib/-/jobs/141105), so plain pytest needs to be used instead.

Test Environment

No manual testing, just verify that the unittest_py3.X jobs actually run on the correct Python version (take a look at the final pipeline output.)

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.