Fixed Cython version to 2.7.3
Fixed Cython version to 2.7.3

Cython 2.8 broke the Appveyor tests.

Stefan-Endres/scipy,lhilt/scipy,aeklant/scipy,rgommers/scipy,jamestwebber/scipy,arokem/scipy,zerothi/scipy,ilayn/scipy,endolith/scipy,vigna/scipy,mdhaber/scipy,gfyoung/scipy,gfyoung/scipy,arokem/scipy,tylerjereddy/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,zerothi/scipy,person142/scipy,WarrenWeckesser/scipy,Eric89GXL/scipy,matthew-brett/scipy,anntzer/scipy,Eric89GXL/scipy,aeklant/scipy,WarrenWeckesser/scipy,gfyoung/scipy,pizzathief/scipy,andyfaff/scipy,andyfaff/scipy,nmayorov/scipy,Stefan-Endres/scipy,gfyoung/scipy,lhilt/scipy,mdhaber/scipy,zerothi/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,aarchiba/scipy,lhilt/scipy,ilayn/scipy,WarrenWeckesser/scipy,ilayn/scipy,jor-/scipy,perimosocordiae/scipy,scipy/scipy,person142/scipy,Eric89GXL/scipy,grlee77/scipy,scipy/scipy,grlee77/scipy,nmayorov/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,tylerjereddy/scipy,arokem/scipy,aeklant/scipy,gertingold/scipy,e-q/scipy,endolith/scipy,jamestwebber/scipy,gertingold/scipy,andyfaff/scipy,Stefan-Endres/scipy,aarchiba/scipy,vigna/scipy,Eric89GXL/scipy,vigna/scipy,aeklant/scipy,aeklant/scipy,anntzer/scipy,jor-/scipy,gertingold/scipy,ilayn/scipy,endolith/scipy,matthew-brett/scipy,Stefan-Endres/scipy,pizzathief/scipy,grlee77/scipy,scipy/scipy,endolith/scipy,WarrenWeckesser/scipy,anntzer/scipy,person142/scipy,tylerjereddy/scipy,jamestwebber/scipy,zerothi/scipy,jamestwebber/scipy,andyfaff/scipy,rgommers/scipy,lhilt/scipy,nmayorov/scipy,nmayorov/scipy,rgommers/scipy,anntzer/scipy,gfyoung/scipy,tylerjereddy/scipy,mdhaber/scipy,vigna/scipy,arokem/scipy,mdhaber/scipy,e-q/scipy,gertingold/scipy,mdhaber/scipy,e-q/scipy,zerothi/scipy,Eric89GXL/scipy,pizzathief/scipy,nmayorov/scipy,pizzathief/scipy,e-q/scipy,pizzathief/scipy,grlee77/scipy,person142/scipy,aarchiba/scipy,jor-/scipy,aarchiba/scipy,jor-/scipy,arokem/scipy,scipy/scipy,ilayn/scipy,Stefan-Endres/scipy,ilayn/scipy,e-q/scipy,endolith/scipy,endolith/scipy,vigna/scipy,gertingold/scipy,andyfaff/scipy,zerothi/scipy,scipy/scipy,aarchiba/scipy,andyfaff/scipy,matthew-brett/scipy,person142/scipy,matthew-brett/scipy,perimosocordiae/scipy,perimosocordiae/scipy,lhilt/scipy,jamestwebber/scipy,jor-/scipy,matthew-brett/scipy,anntzer/scipy,perimosocordiae/scipy,anntzer/scipy,grlee77/scipy,scipy/scipy,mdhaber/scipy,rgommers/scipy,rgommers/scipy,perimosocordiae/scipy