Update all shebangs to python 3

[iwismer]

The readme says that all the python scripts are Python 3 only, but the shebangs in the files do not reflect this, so I updated the shebangs at the tops of the python files to be:

#!/usr/bin/env python3

This allows the files to be run directly on systems where Python 2 is still default.

[jblindsay]

Thanks again Isaac, I should have spotted that myself.