Use astropy.extern.six for import in plotting.py
Use astropy.extern.six for import in plotting.py

I'm guessing that the import of range is intended to be from astropy's copy of six since six isn't listed as a dependency in setup. This PR corrects the import (and is motivated by work on auto-building conda packages for astropy-affiliated packages).
dannygoldstein/sncosmo,kbarbary/sncosmo,sncosmo/sncosmo,kbarbary/sncosmo,sncosmo/sncosmo,kbarbary/sncosmo,sncosmo/sncosmo,dannygoldstein/sncosmo,dannygoldstein/sncosmo