Bump djangorestframework from 3.9.0 to 3.9.1 in /frontend
Bump djangorestframework from 3.9.0 to 3.9.1 in /frontend

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.0 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/3.9.0...3.9.1)
palfrey/kitling,palfrey/kitling,palfrey/kitling