Update tox.ini
Update tox.ini

Adding pytest-xdist to the tox deps so that it can be used with `--parallel`
AustereCuriosity/astropy,tbabej/astropy,saimn/astropy,lpsinger/astropy,stargaser/astropy,StuartLittlefair/astropy,lpsinger/astropy,AustereCuriosity/astropy,kelle/astropy,pllim/astropy,DougBurke/astropy,kelle/astropy,saimn/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,DougBurke/astropy,astropy/astropy,saimn/astropy,MSeifert04/astropy,dhomeier/astropy,larrybradley/astropy,astropy/astropy,larrybradley/astropy,StuartLittlefair/astropy,pllim/astropy,pllim/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,tbabej/astropy,stargaser/astropy,stargaser/astropy,pllim/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,kelle/astropy,tbabej/astropy,mhvk/astropy,funbaker/astropy,DougBurke/astropy,pllim/astropy,saimn/astropy,MSeifert04/astropy,tbabej/astropy,astropy/astropy,funbaker/astropy,larrybradley/astropy,kelle/astropy,astropy/astropy,lpsinger/astropy,saimn/astropy,StuartLittlefair/astropy,tbabej/astropy,joergdietrich/astropy,larrybradley/astropy,kelle/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,dhomeier/astropy,MSeifert04/astropy,AustereCuriosity/astropy,funbaker/astropy,joergdietrich/astropy,dhomeier/astropy,astropy/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,lpsinger/astropy,joergdietrich/astropy,mhvk/astropy,funbaker/astropy,joergdietrich/astropy,stargaser/astropy,mhvk/astropy,joergdietrich/astropy,MSeifert04/astropy,bsipocz/astropy,bsipocz/astropy,aleksandr-bakanov/astropy,mhvk/astropy,DougBurke/astropy,mhvk/astropy,bsipocz/astropy,dhomeier/astropy