Allow implementers to specify their minimum Python version

[tonyandrewmeyer]

Currently, the matrix tests run using Python 3.8. 3.8 is a sensible choice for ops and for 'infrastructure charms' that should support all the bases that currently supported Juju provides, but for 'application charms' it's reasonable to say that a more recent base (and therefore Python) is required.

Ideally, the matrix tests would be able to use either the base or a minimum Python version (e.g. in pyproject.toml) to determine which Python should be use for the compatibility test with the interface.

[PietroPasotti]

in general, I think it'd be handy to start using uv to generate the venvs for the various interface tests. At the all packages for a test suite are installed in a fresh venv, but with the same python.