tox: used final django ~ 4.0 version (#319)

Author: g-as

tox.ini

@@ -10,7 +10,7 @@ deps =
     django2.2: django~=2.2
     django3.1: django~=3.1
     django3.2: django~=3.2
-    django4.0: django==4.0rc1
+    django4.0: django~=4.0
     djangomaster: https://github.com/django/django/archive/master.tar.gz
     .[test]