python_helper: Comment wording
PythonInstalled: Adding a private_site_packages

that live in the `libexec` dir of a `Cellar`.
PythonInstalled: Allow formulae to set/append PYTHONPATH

Improve robustness of `PYTHONPATH` by first unsetting it (during
`satisfy`) so that the `PythonInstalled` can get the `python.version`
and so forth and then, after that, setting the `PYTHONPATH` to our
`global_site_packages`.
In the `python_helper` we append to the `PYTHONPATH` so if that var has
been set in a formula, it is respected.
Brew audit does no longer complain about setting the
`ENV['PYTHONPATH']`.