Edx-opaque-keys

* Moved django pin from django.in to constraint.txt. Upgraded dependencies

* Upgraded dependencies

* Dropped support for Python3.5
* Added support for Django 3.0 & Django 3.1

* setup.py now properly uses requirements files

* Declare support for Python 3.5 and Django 2.2

* Added LibraryLocatorV2 and LibraryUsageLocatorV2
* Added LearningContextKey, UsageKeyV2, and BundleDefinitionLocator
* Added the .is_course helper method