Need to include PYTHONPATH for non-HomeBrew and non-Flask use

[kalebmckale]

How would this feature be useful?

PYTHONPATH has been removed to fix #233 and adjusted to fix #636. However, due to how pipx install functions (using the version of pip inside the new virtualenv it creates), I need to add to PYTHONPATH the path to the customized library that enables my authentication for a specialized PyPI mirror.

Describe the solution you'd like

Ideally, since the only platform that seems to have caused the issue leading to #233 is macOS with Homebrew, one might check for this instead of removing PYTHONPATH unconditionally. Alternatively, one could introduce another command-line option to either add back to PYTHONPATH (possibly via extra_python_paths).

Describe alternatives you've considered

The only workaround I've found so far is tweaking my installed version of pipx to prevent it from removing PYTHONPATH.

[gaborbernat]

PR welcome.