Update RTD config for py39
Update RTD config for py39

mhvk/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,StuartLittlefair/astropy,pllim/astropy,astropy/astropy,astropy/astropy,lpsinger/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,astropy/astropy,larrybradley/astropy,pllim/astropy,pllim/astropy,pllim/astropy,saimn/astropy,saimn/astropy,StuartLittlefair/astropy,lpsinger/astropy,larrybradley/astropy,larrybradley/astropy,StuartLittlefair/astropy,mhvk/astropy,astropy/astropy,saimn/astropy,pllim/astropy,StuartLittlefair/astropy,lpsinger/astropy,mhvk/astropy,larrybradley/astropy,saimn/astropy,mhvk/astropy,lpsinger/astropy