Add missing tox env

Author: apragacz

tox.ini

@@ -1,8 +1,8 @@
 [tox]
 envlist =
-    py{36}-django{20,22},
-    py{37,36}-django{30,31,32},
-    py{39,38,37}-django{40,41,42},
+    py{36}-django{20,21,22},
+    py{36,37}-django{30,31,32},
+    py{37,38,39}-django{40,41,42},
     py{310,311}-django50,
     py{310,311}-djangomaster