Bump djangorestframework from 3.14.0 to 3.15.2

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.14.0 to 3.15.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.14.0...3.15.2)

---
updated-dependencies:
- dependency-name: djangorestframework
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -8,7 +8,7 @@ django-bootstrap-breadcrumbs==0.9.2
 django-extensions==3.2.3
 django-filter==2.3.0
 django-grappelli==3.0.8
-djangorestframework==3.14.0
+djangorestframework==3.15.2
 future~=0.17.1
 gunicorn==20.0.4
 idna==2.9