A user may wish to use two use two brew-installed Python packages
together. For example, one might want to `import numpy` when using
`jupyterlab` or `ptpython`.
Currently, the only ways to do this I'm aware of is with some hacking of
`PYTHONPATH` or the creation of `.pth` files in a formula's prefix.
A better solution is to allow the virtualenvs that `brew` creates to
have access to system site-packages by default, so that `import numpy`
inside `ptpython` or `jupyterlab` just works.
Partially resolvesHomebrew/homebrew-core#76950.