Refactor and document Language::Python.

[reitermarkus]

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Extracted from #8339 for easier review.

[maxim-belkin]

1. Is there a reason why this is not

   if MacOS.version
     PYTHON_VIRTUALENV_URL = ...
   ...
   else
     PYTHON_VIRTUALENV_URL = ...
   ...

2. Below we use MacOS.version but there is no require "os/mac/version" or require "os/mac". Are we sure we don't need it on a Mac?

3. On Linux MacOS.version defaults to NULL, so Linux defaults to using PYTHON_VIRTUALENV_URL_MOJAVE -- I think it's better to make it default to using virtualenv-16.7.4.

[reitermarkus]

I'm not really sure how this should behave.

[Bo98]

PYTHON_VIRTUALENV_URL_MOJAVE purely exists for system Python 2, which I don't know if we even have support for anymore.

It shouldn't be applied to brewed Python 3 at least.

[MikeMcQuaid]

Can try to kill it here.

[maxim-belkin]

Probably not in this PR, but we need to consider using a newer version of virtualenv.
Versions prior to v20.0.0 belong to the "legacy" branch (the latest one being 16.7.10 -- maybe we could use it here even now) and the latest stable is: v20.0.31

https://virtualenv.pypa.io/en/latest/changelog.html

[Bo98]

Using v20 is complicated as it requires a bunch of dependencies.

16.7.10 should be trivial to update to however.