Fix syntax error for python 3
Fix syntax error for python 3

EdwardJKim/nbgrader,jupyter/nbgrader,jupyter/nbgrader,modulexcite/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,alope107/nbgrader,modulexcite/nbgrader,jdfreder/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,MatKallada/nbgrader,jdfreder/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,MatKallada/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,alope107/nbgrader,dementrock/nbgrader,dementrock/nbgrader