Bump djangorestframework from 3.7.4 to 3.9.3

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.7.4 to 3.9.3.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.7.4...3.9.3)

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

setup.py

@@ -51,7 +51,7 @@
         'django==2.0.13',
         'requests==2.20.1',
         'django-axes==1.7.0',
-        'djangorestframework==3.7.4',
+        'djangorestframework==3.9.3',
         'django-reversion==3.0.1',
         'django-compressor==2.2',
         'python-dateutil==2.7.5',