Bump djangorestframework from 3.9.4 to 3.11.0

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.4 to 3.11.0.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.9.4...3.11.0)

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

requirements.txt

@@ -2,7 +2,7 @@ diff-match-patch==20181111
 Django==2.2.8 # pyup: <2.0
 django-filter==2.2.0
 django-import-export==2.0
-djangorestframework==3.9.4
+djangorestframework==3.11.0
 tablib==0.14.0
 beautifulsoup4==4.8.2
 pytz==2019.3