Python 3 compatibility
Python 3 compatibility

Former-commit-id: 6eda4c4ada865f81979f32936eb5e31f0f2ab9d5
gsarma/PyOpenWorm,openworm/PyOpenWorm,hnunner/PyOpenWorm,travs/PyOpenWorm,openworm/PyOpenWorm,gsarma/PyOpenWorm,kaustubhcs/PyOpenWorm