Add __slots__ to product kit AmendmentLineMixin

Closes #12533
(grafted from d987051f942d0ed31faaed7540b8840d51cc767f)

--HG--
branch : 6.0

modules/product_kit/common.py

@@ -425,6 +425,7 @@ def handle_shipment_exception_mixin(prefix):
 
 
 class AmendmentLineMixin:
+    __slots__ = ()
 
     def _apply_line(self, record, line):
         pool = Pool()