Improve version definition for Django 6.0 (#85)

Author: ulgens

tox.ini

@@ -15,7 +15,7 @@ deps =
     dj50: Django==5.0
     dj51: Django==5.1
     dj52: Django==5.2
-    dj60: Django>=6.0a1,<6.1
+    dj60: Django==6.0
     djmain: https://github.com/django/django/tarball/main
 
     django-guardian