fix: remove default_app_config since it is no longer required
In Django 4.1 default_app_config
is not necessary and produces a warning.
Deprecated in 3.2 https://docs.djangoproject.com/en/5.0/releases/3.2/#automatic-appconfig-discovery
Removed in 4.1 https://docs.djangoproject.com/en/5.0/releases/4.1/#features-removed-in-4-1
consider leaving django_mailman3/init.py empty
Edited by Abhilash Raj