Starting geant4 10.6.2 g4py can not be built separately
http://geant4-data.web.cern.ch/geant4-data/ReleaseNotes/Patch4.10.6-2.txt
Also, it appears that g4py itself is now deprecated, it was moved
to environments/g4py/tests/g4pytest in the source distribution. The only
remaining imported module is Geant4, hence python package name
`pythonPackages.geant4`, the capitalization matches the one of the non-python
attribute.