TST: Pin Sphinx so linkcheck works
TST: Pin Sphinx so linkcheck works

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