Update minimum python version in install docs [docs only]
Update minimum python version in install docs [docs only]

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