Geant4 +Python: fix module load (#17046)
Geant4 +Python: fix module load (#17046)

Adding `extends('python')` makes sure that a

```
spack load -r geant4 +python
python -c "import Geant4"
```

find the module.
iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack