add mpmath to optional conda dependencies
add mpmath to optional conda dependencies

It's an optional dependency for my new code. I need to add it to make sure new code is covered.
astropy/astropy,mhvk/astropy,tbabej/astropy,bsipocz/astropy,mhvk/astropy,MSeifert04/astropy,bsipocz/astropy,dhomeier/astropy,astropy/astropy,lpsinger/astropy,dhomeier/astropy,pllim/astropy,pllim/astropy,joergdietrich/astropy,mhvk/astropy,tbabej/astropy,lpsinger/astropy,lpsinger/astropy,saimn/astropy,MSeifert04/astropy,DougBurke/astropy,mhvk/astropy,stargaser/astropy,stargaser/astropy,aleksandr-bakanov/astropy,MSeifert04/astropy,larrybradley/astropy,larrybradley/astropy,lpsinger/astropy,tbabej/astropy,StuartLittlefair/astropy,kelle/astropy,joergdietrich/astropy,joergdietrich/astropy,StuartLittlefair/astropy,dhomeier/astropy,bsipocz/astropy,joergdietrich/astropy,astropy/astropy,DougBurke/astropy,pllim/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,saimn/astropy,kelle/astropy,stargaser/astropy,StuartLittlefair/astropy,DougBurke/astropy,bsipocz/astropy,astropy/astropy,kelle/astropy,aleksandr-bakanov/astropy,funbaker/astropy,lpsinger/astropy,saimn/astropy,AustereCuriosity/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,saimn/astropy,DougBurke/astropy,kelle/astropy,larrybradley/astropy,AustereCuriosity/astropy,pllim/astropy,kelle/astropy,mhvk/astropy,AustereCuriosity/astropy,joergdietrich/astropy,funbaker/astropy,AustereCuriosity/astropy,saimn/astropy,tbabej/astropy,StuartLittlefair/astropy,stargaser/astropy,funbaker/astropy,astropy/astropy,funbaker/astropy,aleksandr-bakanov/astropy,pllim/astropy,tbabej/astropy,MSeifert04/astropy,dhomeier/astropy,larrybradley/astropy,dhomeier/astropy