Ensure our intermediate AppConfig class is not selected s default

aleksis/core/__init__.py

@@ -6,5 +6,3 @@ try:
     __version__ = metadata.distribution("AlekSIS-Core").version
 except Exception:
     __version__ = "unknown"
-
-default_app_config = "aleksis.core.apps.CoreConfig"

aleksis/core/util/apps.py

@@ -20,8 +20,13 @@ if TYPE_CHECKING:
 class AppConfig(django.apps.AppConfig):
     """An extended version of DJango's AppConfig container."""
 
+    default = False
     default_auto_field = "django.db.models.BigAutoField"
 
+    def __init_subclass__(cls):
+        super().__init_subclass__()
+        cls.default = True
+
     def ready(self):
         super().ready()