In our sitecustomize.py we instruct python to exit with an error message
if the PYTHONPATH is pointing to a wrong homebrew site-packages dir.
With wrong meaning another major python version.
If you set the PYTHONPATH to include
$(brew --prefix)/lib/python2.7/site-packages
and start python3, it may pick up modules from there, wich can result
in errors for non pure python modules (such as PyQt).