Add pypy3 to Travis test matrix
Add pypy3 to Travis test matrix

harlowja/networkx,wasade/networkx,jakevdp/networkx,ghdk/networkx,sharifulgeo/networkx,farhaanbukhsh/networkx,tmilicic/networkx,kernc/networkx,debsankha/networkx,harlowja/networkx,jakevdp/networkx,blublud/networkx,bzero/networkx,ghdk/networkx,kernc/networkx,aureooms/networkx,debsankha/networkx,sharifulgeo/networkx,dmoliveira/networkx,yashu-seth/networkx,ionanrozenfeld/networkx,dmoliveira/networkx,SanketDG/networkx,RMKD/networkx,dhimmel/networkx,aureooms/networkx,chrisnatali/networkx,jni/networkx,JamesClough/networkx,nathania/networkx,jni/networkx,farhaanbukhsh/networkx,ionanrozenfeld/networkx,harlowja/networkx,aureooms/networkx,jfinkels/networkx,ltiao/networkx,beni55/networkx,dmoliveira/networkx,ghdk/networkx,dhimmel/networkx,kernc/networkx,jakevdp/networkx,bzero/networkx,cmtm/networkx,blublud/networkx,chrisnatali/networkx,NvanAdrichem/networkx,jcurbelo/networkx,debsankha/networkx,sharifulgeo/networkx,dhimmel/networkx,OrkoHunter/networkx,nathania/networkx,andnovar/networkx,farhaanbukhsh/networkx,nathania/networkx,Sixshaman/networkx,michaelpacer/networkx,bzero/networkx,RMKD/networkx,goulu/networkx,chrisnatali/networkx,ionanrozenfeld/networkx,RMKD/networkx,jni/networkx,blublud/networkx