Fix Python 2.6 compatibility for #4517
Fix Python 2.6 compatibility for #4517

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