Bump djangorestframework from 3.4.7 to 3.15.2

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.4.7 to 3.15.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.4.7...3.15.2)

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

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

requirements.txt

@@ -27,7 +27,7 @@ django-formtools==2.1
 django-cors-headers==1.3.1
 
 ### API related
-djangorestframework==3.4.7
+djangorestframework==3.15.2
 
 ### Django related registration
 # The registration app is a little out of date - get it from the repo direct....