Tests: Update for Django 3.0 release

Author: medmunds

tox.ini

@@ -26,7 +26,8 @@ deps =
     django20: django~=2.0.0
     django21: django~=2.1.0
     django22: django~=2.2.0
-    django30: django>=3.0a1
+    django30: django~=3.0.0
+    django31: django>=3.1a1
     djangoMaster: https://github.com/django/django/tarball/master
     old_urllib3: urllib3<1.25
     # testing dependencies (duplicates setup.py tests_require, less optional extras):