Always return a timedelta for product supply_period

issue6570
review35451002

product.py

@@ -1,5 +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.
+from datetime import timedelta
 
 from trytond.pool import Pool, PoolMeta
 
@@ -14,4 +15,4 @@ class Product:
         'Return the supply period for the product'
         pool = Pool()
         Configuration = pool.get('production.configuration')
-        return Configuration(1).supply_period or 0
+        return Configuration(1).supply_period or timedelta(0)