also allow none in differentialattribute
also allow none in differentialattribute

dhomeier/astropy,astropy/astropy,larrybradley/astropy,lpsinger/astropy,StuartLittlefair/astropy,larrybradley/astropy,stargaser/astropy,lpsinger/astropy,lpsinger/astropy,astropy/astropy,pllim/astropy,pllim/astropy,stargaser/astropy,StuartLittlefair/astropy,saimn/astropy,saimn/astropy,aleksandr-bakanov/astropy,MSeifert04/astropy,dhomeier/astropy,astropy/astropy,pllim/astropy,dhomeier/astropy,mhvk/astropy,mhvk/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,pllim/astropy,mhvk/astropy,saimn/astropy,MSeifert04/astropy,astropy/astropy,aleksandr-bakanov/astropy,saimn/astropy,StuartLittlefair/astropy,stargaser/astropy,lpsinger/astropy,larrybradley/astropy,saimn/astropy,larrybradley/astropy,larrybradley/astropy,mhvk/astropy,mhvk/astropy,MSeifert04/astropy,lpsinger/astropy,pllim/astropy,dhomeier/astropy,astropy/astropy,dhomeier/astropy,StuartLittlefair/astropy,MSeifert04/astropy,stargaser/astropy