Bump djangorestframework from 3.14.0 to 3.15.2 in /tests

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-version: 3.15.2
  dependency-type: direct:production
...

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

Author: dependabot[bot]

tests/requirements.txt

@@ -1,3 +1,3 @@
 -r ../requirements.txt
-djangorestframework==3.14.0
+djangorestframework==3.15.2
 model_mommy==2.0.0