Add Model as base for ExtensibleModel

aleksis/core/mixins.py

@@ -17,7 +17,7 @@ from rules.contrib.admin import ObjectPermissionsModelAdmin
 
 
 @reversion.register()
-class ExtensibleModel:
+class ExtensibleModel(models.Model):
     """Base model for all objects in AlekSIS apps.
 
     This base model ensures all objects in AlekSIS apps fulfill the