Update Django 4.0 version in tox.ini to RC1

Author: pauloxnet

tox.ini

@@ -12,7 +12,7 @@ deps =
     dj22: Django==2.2.*
     dj31: Django==3.1.*
     dj32: Django>=3.2,<4.0
-    dj40: Django>=4.0b1,<4.1
+    dj40: Django>=4.0rc1,<4.1
     sqlite: mock
     postgresql: psycopg2-binary
     postgis: psycopg2-binary