Bump djangorestframework from 3.14.0 to 3.15.0

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.14.0 to 3.15.0.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.14.0...3.15.0)

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

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

Author: dependabot[bot]