base: rework the way we use pyenv to make it more self sufficient

The Python installed by pyenv were not working out of the box. I figured out that we needed to set the PYENV_ROOT variable globally for shims to find the Python version lucky enough to be "enable".

However while wrestling I learned too much about pyenv and shims to be comfortable with them. So I move toward making the python installed by pyenv directly available globally close to want installing multiple python version through debian would do. This make for a more "standard" installation that should be move robust.

I also moved to pipx for most of the utility installation.

The pytype setup is still a it weird because the way it is called in the CI is still weird, cleaning that up would be nice.

I went for installing most things are root, to have a more "standard" environment and detect if some code tries to get a bit too comfortable with the system installs.

I also restore a working "python3" install from Debian at it provide a 3.11 working fine.

I tried to document most quirk for future readers.