Add Python 3.9 support

tox.ini

@@ -1,8 +1,9 @@
 [tox]
 envlist =
     py27-django111
-    py{35,36,37,38}-django{111,20,21,22}
-    py{36,37,38}-django{30,31}
+    py{35,36,37,38}-django{111,20,21}
+    py{35,36,37,38,39}-django22
+    py{36,37,38,39}-django{30,31}
     lint
 
 [gh-actions]
@@ -12,6 +13,7 @@ python =
     3.6: py36
     3.7: py37
     3.8: py38
+    3.9: py39
 
 [testenv]
 commands = ./manage.py test --no-input {posargs}