Update nbgrader/api.py
Update nbgrader/api.py

Co-Authored-By: sigurdurb <a01938c980e651145dbe3ecc380defdb95e31ab1@gmail.com>
jhamrick/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader