Bump djangorestframework from 3.14.0 to 3.15.1 in /requirements

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] <[email protected]>

requirements/base.txt

@@ -40,7 +40,7 @@ gunicorn==21.2.0
 # Django Rest Framework
 # ------------------------------------------------------------------------------
 # https://github.com/encode/django-rest-framework
-djangorestframework==3.15.0
+djangorestframework==3.15.1
 # https://github.com/adamchainz/django-cors-headers
 django-cors-headers==4.3.1
 # https://github.com/alanjds/drf-nested-routers