Do not copy shipment costs when copying sale

Closes #13384
(grafted from 37358bba5fdb2a4a092abd5e75113917a00d1b94)

--HG--
branch : 7.0

modules/sale_shipment_cost/sale.py

@@ -395,6 +395,12 @@ class Sale(metaclass=PoolMeta):
             shipment.carrier = self.carrier
         return shipment
 
+    @classmethod
+    def copy(cls, sales, default=None):
+        default = default.copy() if default is not None else {}
+        default.setdefault('shipment_costs', None)
+        return super().copy(sales, default=default)
+
 
 class Line(metaclass=PoolMeta):
     __name__ = 'sale.line'