ci: manager: python3.7 unit tests removed

Python3.7 is at end-of-life (EOL) and it started to break dependencies (e.g. supervisord). So from now on we will rely only on pkg tests.

I also tried adding unit tests for python3.12 but it fails on poetry install. At least I added python3.12 to the docker file and updated poetry installation with pipx.

Locally everything seems to work as it should. I will push the new image to the registry later after.