Run python-modernize on codejail

Once tests are passing under Python 2.7 in Jenkins, run python-modernize and isort on codejail's Python files (an early attempt at this was made here). If the Python 3.5 and 3.6 tests start passing after this, update the trove classifiers in setup.py appropriately and update openedx.yaml to claim OEP-7 support. Otherwise, create new tickets for fixing any tests that still fail.