remove DeprecatedEdxPlatformImportWarning

[BbrSofiane]

Remove warnings when from staff_graded when running:

manage.py lms createsuperuser

2020-11-23 14:28:11,450 WARNING 240810 [py.warnings] [user None] [ip None] warnings.py:109 - /edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/staff_graded/staff_graded.py:27: DeprecatedEdxPlatformImportWarning: Importing course_modes instead of common.djangoapps.course_modes is deprecated
  from course_modes.models import CourseMode

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