Fixes failing mypy test by pinning pytest-mypy to 0.4.2 as suggested in issue #1328
Fixes failing mypy test by pinning pytest-mypy to 0.4.2 as suggested in issue #1328

jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jupyter/nbgrader