make symlinking python3->python optional (#7960)
make symlinking python3->python optional (#7960)

* Make symlinking python3->python optional
* Added reference to PEP 394
tmerrick1/spack,tmerrick1/spack,LLNL/spack,iulian787/spack,krafczyk/spack,matthiasdiener/spack,mfherbst/spack,matthiasdiener/spack,mfherbst/spack,krafczyk/spack,matthiasdiener/spack,iulian787/spack,iulian787/spack,matthiasdiener/spack,mfherbst/spack,mfherbst/spack,tmerrick1/spack,LLNL/spack,LLNL/spack,iulian787/spack,tmerrick1/spack,krafczyk/spack,krafczyk/spack,tmerrick1/spack,matthiasdiener/spack,LLNL/spack,krafczyk/spack,iulian787/spack,mfherbst/spack,LLNL/spack