update setup.py to reflect changes in astropy core
update setup.py to reflect changes in astropy core

dannygoldstein/sncosmo,sncosmo/sncosmo,kbarbary/sncosmo,kbarbary/sncosmo,sncosmo/sncosmo,dannygoldstein/sncosmo,sncosmo/sncosmo,dannygoldstein/sncosmo,kbarbary/sncosmo