Add round_price

We add a common method to round prices that are stored using the price digits.
It is useless to use the digits from the field because it can not be changed in
any other way than by configuration.

issue9146
review299521002

product.py

@@ -1,7 +1,5 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
-from decimal import Decimal
-
 from sql import Column, Literal, Window
 from sql.aggregate import Max
 from sql.conditionals import Coalesce
@@ -11,6 +9,8 @@ from trytond.model import ModelView, ModelSQL, fields
 from trytond.pool import Pool, PoolMeta
 from trytond.transaction import Transaction
 
+from trytond.modules.product import round_price
+
 
 def convert_from(table, tables):
     right, condition = tables[None]
@@ -48,8 +48,7 @@ class Product(metaclass=PoolMeta):
                 ], limit=1, order=[('date', 'DESC')])
         if records:
             record, = records
-            return record.cost_price.quantize(
-                Decimal(str(10. ** -self.__class__.cost_price.digits[1])))
+            return round_price(record.cost_price)
 
 
 class CostPrice(metaclass=PoolMeta):