add price_digits() to mixin.py

mixin.py

@@ -10,7 +10,15 @@ from trytond.exceptions import UserError
 from trytond.modules.tryton6_backport.i18n import gettext
 from cached_property import cached_property
 from trytond.pool import Pool
-from trytond.modules.product import round_price
+from trytond.modules.product import price_digits
+
+
+def round_price(value, rounding=None):
+    "Round price using the price digits"
+    if isinstance(value, int):
+        return Decimal(value)
+    return value.quantize(
+        Decimal(1) / 10 ** price_digits[1], rounding=rounding)
 
 
 class EdocumentMixin(object):