Add Python 3.10 support

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     py{37,38,39}-django22
-    py{37,38,39}-django{31,32}
+    py{37,38,39,310}-django{31,32}
     lint
     coverage
 
@@ -10,6 +10,7 @@ python =
     3.7: py37
     3.8: py38
     3.9: py39
+    3.10: py310
 
 [testenv]
 commands = ./manage.py test --no-input {posargs}