add Pillow to doc requirements
add Pillow to doc requirements

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