Bump djangorestframework from 3.9.3 to 3.9.4

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.9.3 to 3.9.4.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.9.3...3.9.4)

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

requirements/base.txt

@@ -6,7 +6,7 @@ django-csp==3.5
 django-csp-reports==1.2
 django-crispy-forms==1.7.2
 django-filter==2.1.0
-djangorestframework==3.9.3
+djangorestframework==3.9.4
 django-secure==1.0.1
 django-toolbelt==0.0.1
 gunicorn==19.9.0