Update nbgrader/plugins/export.py
Update nbgrader/plugins/export.py

Co-Authored-By: Jessica B. Hamrick <e9b4923718a9b6bad2a5bd81d1aa9e369ef6ed7f@gmail.com>
jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader