change rtd requirements to stable astropy
change rtd requirements to stable astropy

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