Correct python3 use of super in astropy.units.
Correct python3 use of super in astropy.units.

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