Add support for Django REST Framework 3.9

As with 3.7 and 3.8, there are no breaking changes we need to be
concerned with here.

Signed-off-by: Stephen Finucane <stephen@that.guru>
(backported from commit fab5571
 to not include changes to requirements-*.txt, just release note
 and tox.ini changes.)
Signed-off-by: Daniel Axtens <dja@axtens.net>

Author: stephenfin

releasenotes/notes/django-rest-framework-3-9-0afb78322dd82367.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    `Django REST Framework 3.9
+    <https://www.django-rest-framework.org/community/3.9-announcement/>`_ is now
+    supported.

tox.ini

@@ -12,7 +12,7 @@ deps =
     django111: django>=1.11,<2.0
     django{18,19}: djangorestframework>=3.4,<3.7
     django110: djangorestframework>=3.4,<3.9
-    django111: djangorestframework>=3.6,<3.9
+    django111: djangorestframework>=3.6,<3.10
     django18: django-filter>=1.0,<1.1
     django{19,110,111}: django-filter>=1.0,<1.2
 setenv =