INCR-328 python3 compatibility

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/tests/test_course_listing.py

cms/djangoapps/contentstore/tests/test_courseware_index.py

cms/djangoapps/contentstore/tests/test_export_git.py

cms/djangoapps/contentstore/tests/test_i18n.py

cms/djangoapps/contentstore/tests/test_orphan.py

cms/djangoapps/contentstore/tests/test_permissions.py

cms/djangoapps/contentstore/tests/test_transcripts_utils.py

cms/djangoapps/contentstore/tests/test_users_default_role.py

cms/djangoapps/contentstore/tests/test_video_utils.py