Bump djangorestframework from 3.8.2 to 3.9.1 in /requirements
Bump djangorestframework from 3.8.2 to 3.9.1 in /requirements

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.8.2 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/3.8.2...3.9.1)
openstax/openstax-cms,Connexions/openstax-cms,openstax/openstax-cms,openstax/openstax-cms,Connexions/openstax-cms,openstax/openstax-cms