tests: check min required python version with vermin (#14289)

This commit removes the `python_version.py` unit test module
and the vendored dependencies `pyqver2.py` and `pyqver3.py`.
It substitutes them with an equivalent check done using
`vermin` that is run as a separate workflow via Github Actions.

This allows us to delete 2 vendored dependencies that are unmaintained
and substitutes them with a maintained tool.

Also, updates the list of vendored dependencies.