Ask on Python discourse how --system-site-packages and PYTHONPATH interact

[matthewfeickert]

This is a TODO item: Find out why in some circumstances (e.g. an ATLAS AnalysisBase release) the use of --system-site-packages is sufficient even though precedence on PYTHONPATH of the virtual environment is low, but in other circumstances (e.g. LCG view you need to hack back PYTHONPATH to have control). Basically understand how --system-site-packages works and exactly how PYTHONPATH works with pip.