Bump djangorestframework from 3.6.3 to 3.9.1
Bump djangorestframework from 3.6.3 to 3.9.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.6.3 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/3.6.3...3.9.1)
BirkbeckCTP/janeway,BirkbeckCTP/janeway,BirkbeckCTP/janeway,BirkbeckCTP/janeway