greg.signals: allow access to protected member _changed_fields

Fixes: GREG-16

greg/signals.py

@@ -75,7 +75,9 @@ def add_changed_fields_callback(sender, instance, raw, *args, **kwargs):
     changed = instance.is_dirty()
     if not changed:
         return
-    instance._changed_fields = list(instance.get_dirty_fields().keys())
+    instance._changed_fields = list(
+        instance.get_dirty_fields().keys()
+    )  # pylint: disable=protected-access
 
 
 @receiver(models.signals.post_save, dispatch_uid="save_notification_callback")