Configurable venv manager

[Lattay]

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• Yes

Is your feature request related to a problem? Please describe.

On my system python-venv is not installed and I cannot install it. This breaks the installation of various packages.

Describe the solution you'd like

I cannot install venv, but I can install virtualenv with pip install virtualenv, then replace calls to python -m venv with python -m virtualenv in mason.

Lattay@dd137f6

Describe potential alternatives you've considered

Changing the python environment, possible but cumbersome and anoying.

Additional context

If this feature is desirable for mason, I'll implement it and make a PR.

Obviously, my hardcoding is not a good fix to upstream, so I open this issue to discuss whether it should be a config option, in which case, I ask where it should gom, or if mason should automatically try both ven and virtualenv.