Bump djangorestframework from 3.14.0 to 3.15.1

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.14.0 to 3.15.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.14.0...3.15.1)

---
updated-dependencies:
- dependency-name: djangorestframework
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>