Bump djangorestframework from 3.9.4 to 3.11.2

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.4 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.9.4...3.11.2)

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

Author: dependabot[bot]

requirements_dev.txt

@@ -4,7 +4,7 @@ pytest-cov==2.5.1
 falcon==3.0.0
 flask
 django==2.2.20
-djangorestframework==3.9.4
+djangorestframework==3.11.2
 requests==2.22.0
 responses==0.10.12
 webob