remove use of mutable arguments as defaults
remove use of mutable arguments as defaults

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