Bump djangorestframework from 3.7.7 to 3.9.1 in /requirements

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.7.7 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.7.7...3.9.1)

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

requirements/base.txt

@@ -3,4 +3,4 @@ Xblock==1.2.9
 
 # edx-submissions
 edx-submissions==3.0.2
-djangorestframework==3.7.7 # Now needed by edx-submissions
+djangorestframework==3.9.1 # Now needed by edx-submissions