Fixed error on Python 2
Fixed error on Python 2

ktaneishi/deepchem,miaecle/deepchem,Agent007/deepchem,miaecle/deepchem,ktaneishi/deepchem,ktaneishi/deepchem,deepchem/deepchem,Agent007/deepchem,deepchem/deepchem,lilleswing/deepchem,Agent007/deepchem,peastman/deepchem,peastman/deepchem,lilleswing/deepchem,miaecle/deepchem,lilleswing/deepchem