python: use built-in venv instead of virtualenv when installing formulae

[dtrodrigues]

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This PR uses python's built-in venv to install formulae instead of the external virtualenv, which has dependencies and needs to be bootstrapped. The dependencies could also theoretically differ based on Python version.

Current caveat: venv uses ensurepip to install pip and setuptools using the wheels that are bundled with cpython, even if a newer one is installed, which is 20.2.3 for pip and 49.2.1 for setuptools in Python 3.9.1: https://github.com/python/cpython/blob/1e5d33e9b9b8631b36f061103a30208b206fd03a/Lib/ensurepip/__init__.py#L16-L18, https://github.com/python/cpython/tree/1e5d33e9b9b8631b36f061103a30208b206fd03a/Lib/ensurepip/_bundled

Other downstream package managers patch ensurepip to point to their locally managed pip and setuptools: https://src.fedoraproject.org/rpms/python3.9/blob/2084e749c71fe494a9a4ed76ce9c2d149b76868c/f/00189-use-rpm-wheels.patch, so that's a possible solution to ensure the temporary installation venv is using the latest versions that Homebrew knows about vs either using a known outdated version or blindly using the latest available in PyPI.

Marking as a draft for now and can look into writing new tests if it's decided to go in this direction.

Follow-up to #9435.

[BrewTestBot]

Review period will end on 2020-12-17 at 17:41:06 UTC.

[BrewTestBot]

Review period will end on 2020-12-17 at 17:41:06 UTC.

[MikeMcQuaid]

I'd be happy to see us go in this direction. Nice work @dtrodrigues!

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Nice work, thanks @dtrodrigues!