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

production.py

@@ -1,7 +1,6 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 import hashlib
-from decimal import Decimal
 from itertools import groupby
 
 from trytond.i18n import gettext
@@ -9,6 +8,8 @@ from trytond.model import ModelView, Workflow, fields
 from trytond.pool import PoolMeta, Pool
 from trytond.pyson import Eval, Bool, If
 
+from trytond.modules.product import round_price
+
 from .exceptions import PurchaseWarning
 
 
@@ -123,8 +124,7 @@ class Production(metaclass=PoolMeta):
                 cost += Currency.compute(
                     line.purchase.currency, line.amount,
                     self.company.currency, round=False)
-        digits = self.__class__.cost.digits
-        return cost.quantize(Decimal(str(10 ** -digits[1])))
+        return round_price(cost)
 
     @classmethod
     @ModelView.button