Only build docs on python 3
Only build docs on python 3

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