Bump djangorestframework

Bumps [djangorestframework](https://github.com/encode/django-rest-framework) from 3.4.7 to 3.9.1.
- [Release notes](https://github.com/encode/django-rest-framework/releases)
- [Commits](encode/django-rest-framework@3.4.7...3.9.1)

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

samples/stamp/atos/functional/template/citygo/requirements.txt

@@ -1,7 +1,7 @@
 Django==1.10.2
 django-allauth==0.27.0
 django-rest-auth==0.8.1
-djangorestframework==3.4.7
+djangorestframework==3.9.1
 httplib2==0.9.2
 oauth2==1.9.0.post1
 oauthlib==2.0.0