Bump djangorestframework from 3.9.1 to 3.11.2 in /requirements

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.1 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](https://github.com/encode/django-rest-framework/compare/3.9.1...3.11.2)

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

requirements/requirements.txt

@@ -1,7 +1,7 @@
 django==2.2.18
 django-haystack==2.8.1
 django-filter==2.0.0
-djangorestframework==3.9.1
+djangorestframework==3.11.2
 dj-database-url==0.5.0
 django-braces==1.14.0
 django-crispy-forms==1.7.2