pip+virtualenv installation fix. closes #367
pip+virtualenv installation fix. closes #367

For unknown reasons (probably a mix between pip, setuptools and distutils invocation), our extension source are automatically replaced from pyx to c, and then, Cython is not doing its work.
So after invocation of the Extension.__init__, force again the source field.

niavlys/kivy,Farkal/kivy,inclement/kivy,JohnHowland/kivy,edubrunaldi/kivy,iamutkarshtiwari/kivy,CuriousLearner/kivy,VinGarcia/kivy,arcticshores/kivy,cbenhagen/kivy,hansent/kivy,habibmasuro/kivy,xiaoyanit/kivy,bob-the-hamster/kivy,yoelk/kivy,Shyam10/kivy,kivy/kivy,KeyWeeUsr/kivy,darkopevec/kivy,kived/kivy,Farkal/kivy,tony/kivy,Cheaterman/kivy,jkankiewicz/kivy,ernstp/kivy,matham/kivy,KeyWeeUsr/kivy,tony/kivy,jegger/kivy,kived/kivy,vitorio/kivy,el-ethan/kivy,xpndlabs/kivy,janssen/kivy,adamkh/kivy,ehealthafrica-ci/kivy,akshayaurora/kivy,janssen/kivy,thezawad/kivy,inclement/kivy,kivatu/kivy-bak,Davideddu/kivy-forkedtouch,el-ethan/kivy,bionoid/kivy,autosportlabs/kivy,dirkjot/kivy,arlowhite/kivy,matham/kivy,adamkh/kivy,andnovar/kivy,darkopevec/kivy,angryrancor/kivy,LogicalDash/kivy,adamkh/kivy,youprofit/kivy,janssen/kivy,Davideddu/kivy-forkedtouch,Ramalus/kivy,gonzafirewall/kivy,akshayaurora/kivy,darkopevec/kivy,eHealthAfrica/kivy,vipulroxx/kivy,vipulroxx/kivy,JohnHowland/kivy,dirkjot/kivy,cbenhagen/kivy,jegger/kivy,Cheaterman/kivy,arcticshores/kivy,mSenyor/kivy,bionoid/kivy,LogicalDash/kivy,bliz937/kivy,matham/kivy,bhargav2408/kivy,angryrancor/kivy,xpndlabs/kivy,zennobjects/kivy,kivatu/kivy-bak,Shyam10/kivy,rnixx/kivy,aron-bordin/kivy,kivy/kivy,ernstp/kivy,JohnHowland/kivy,aron-bordin/kivy,manashmndl/kivy,dirkjot/kivy,kivatu/kivy-bak,jkankiewicz/kivy,hansent/kivy,inclement/kivy,youprofit/kivy,eHealthAfrica/kivy,ehealthafrica-ci/kivy,viralpandey/kivy,edubrunaldi/kivy,zennobjects/kivy,Cheaterman/kivy,Farkal/kivy,jkankiewicz/kivy,arcticshores/kivy,manashmndl/kivy,wangjun/kivy,youprofit/kivy,mSenyor/kivy,yoelk/kivy,yoelk/kivy,Ramalus/kivy,jegger/kivy,gonzafirewall/kivy,matham/kivy,CuriousLearner/kivy,xpndlabs/kivy,CuriousLearner/kivy,arcticshores/kivy,iamutkarshtiwari/kivy,andnovar/kivy,mSenyor/kivy,darkopevec/kivy,autosportlabs/kivy,niavlys/kivy,ehealthafrica-ci/kivy,rafalo1333/kivy,viralpandey/kivy,dirkjot/kivy,vipulroxx/kivy,jkankiewicz/kivy,rnixx/kivy,denys-duchier/kivy,ehealthafrica-ci/kivy,andnovar/kivy,manashmndl/kivy,niavlys/kivy,manthansharma/kivy,bhargav2408/kivy,bliz937/kivy,angryrancor/kivy,ernstp/kivy,arlowhite/kivy,habibmasuro/kivy,thezawad/kivy,LogicalDash/kivy,aron-bordin/kivy,kivy/kivy,habibmasuro/kivy,aron-bordin/kivy,KeyWeeUsr/kivy,tony/kivy,jffernandez/kivy,denys-duchier/kivy,jehutting/kivy,niavlys/kivy,manthansharma/kivy,vipulroxx/kivy,akshayaurora/kivy,rafalo1333/kivy,Farkal/kivy,bob-the-hamster/kivy,bhargav2408/kivy,xiaoyanit/kivy,jehutting/kivy,Shyam10/kivy,Davideddu/kivy-forkedtouch,autosportlabs/kivy,LogicalDash/kivy,bob-the-hamster/kivy,VinGarcia/kivy,gonzafirewall/kivy,jegger/kivy,yoelk/kivy,kived/kivy,zennobjects/kivy,ernstp/kivy,rnixx/kivy,hansent/kivy,angryrancor/kivy,jehutting/kivy,arlowhite/kivy,cbenhagen/kivy,gonzafirewall/kivy,Davideddu/kivy-forkedtouch,JohnHowland/kivy,jffernandez/kivy,VinGarcia/kivy,bionoid/kivy,Cheaterman/kivy,eHealthAfrica/kivy,vitorio/kivy,zennobjects/kivy,eHealthAfrica/kivy,denys-duchier/kivy,jffernandez/kivy,jffernandez/kivy,bob-the-hamster/kivy,MiyamotoAkira/kivy,KeyWeeUsr/kivy,bliz937/kivy,manthansharma/kivy,viralpandey/kivy,manthansharma/kivy,xiaoyanit/kivy,Ramalus/kivy,el-ethan/kivy,adamkh/kivy,wangjun/kivy,wangjun/kivy,rafalo1333/kivy,kivatu/kivy-bak,MiyamotoAkira/kivy,Shyam10/kivy,iamutkarshtiwari/kivy,hansent/kivy,thezawad/kivy,MiyamotoAkira/kivy,janssen/kivy,wangjun/kivy,denys-duchier/kivy,edubrunaldi/kivy,vitorio/kivy,MiyamotoAkira/kivy,bionoid/kivy