Bump djangorestframework from 3.11.0 to 3.11.2 in /backend

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.11.0 to 3.11.2.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.11.0...3.11.2)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -3,7 +3,7 @@ certifi==2019.11.28
 chardet==3.0.4
 coreschema==0.0.4
 Django==3.0.7
-djangorestframework==3.11.0
+djangorestframework==3.11.2
 idna==2.9
 itypes==1.1.0
 Jinja2==2.11.1