Add Django 4.0a1 to tox.ini (#1508)

* Add Django 4.0a1 to tox.ini
* Rewrite the tox.ini matrix

Thanks @felixmm!

Author: matthiask

tox.ini

@@ -3,16 +3,16 @@ envlist =
     docs
     style
     packaging
-    py{36,37}-dj{22,31,32}-sqlite
-    py{38,39}-dj{22,31,32,main}-sqlite
-    py{36,37,38,39}-dj{22,31,32}-{postgresql,mysql}
-    py{310}-dj{32,main}-{sqlite,postgresql,mysql}
+    py{36,37}-dj{22,31,32}-{sqlite,postgresql,mysql}
+    py{38,39}-dj{22,31,32,40,main}-{sqlite,postgresql,mysql}
+    py{310}-dj{40,main}-{sqlite,postgresql,mysql}
 
 [testenv]
 deps =
     dj22: Django==2.2.*
     dj31: Django==3.1.*
-    dj32: Django>=3.2a1,<4.0
+    dj32: Django>=3.2,<4.0
+    dj40: Django>=4.0a1,<4.1
     sqlite: mock
     postgresql: psycopg2-binary
     mysql: mysqlclient