Test with Django REST Framework 3.9

tox.ini

@@ -1,9 +1,9 @@
 [tox]
 # https://docs.djangoproject.com/en/dev/faq/install/#what-python-version-can-i-use-with-django
 envlist =
-    py{27,34,35,36}-django111-drf{37,38},
-    py{34,35,36,37}-django20-drf{37,38},
-    py{35,36,37}-django21-drf{37,38},
+    py{27,34,35,36}-django111-drf{37,38,39},
+    py{34,35,36,37}-django20-drf{37,38,39},
+    py{35,36,37}-django21-drf{37,38,39},
     djmaster, lint, docs
 
 [testenv]
@@ -14,6 +14,7 @@ deps =
 
     drf37: djangorestframework>=3.7.7,<3.8
     drf38: djangorestframework>=3.8.0,<3.9
+    drf39: djangorestframework>=3.9,<3.10
 
     # test with the latest build of django-rest-framework to get early warning of compatibility issues
     djmaster: https://github.com/encode/django-rest-framework/archive/master.tar.gz