Set up tox to run tests against Django 3.1

Author: Photonios

tox.ini

@@ -1,12 +1,13 @@
 [tox]
-envlist = py36-dj{20,21,22,30}, py37-dj{20,21,22,30}, py38-dj{20,21,22,30}, py39-dj{21,22,30}
+envlist = py36-dj{20,21,22,30,31}, py37-dj{20,21,22,30,31}, py38-dj{20,21,22,30,31}, py39-dj{21,22,30,31}
 
 [testenv]
 deps =
     dj20: Django>=2.0,<2.1
     dj21: Django>=2.1,<2.2
     dj22: Django>=2.2,<2.3
     dj30: Django>=3.0,<3.0.2
+    dj31: Django>=3.1,<3.2
     .[test]
 setenv =
     DJANGO_SETTINGS_MODULE=settings