Add back the remove djmaster tox test

Author: samiashi

tox.ini

@@ -26,6 +26,7 @@ deps =
     dj4.2: Django>=4.2,<4.3
     dj5.1: Django>=5.1,<5.2
     dj5.2: Django>=5.2,<5.3
+    djmaster: https://github.com/django/django/archive/refs/heads/main.zip
 
 allowlist_externals = *