Add missing with_statement from __future__

tests/test_stock.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 #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 __future__ import with_statement
 
 import sys, os
 DIR = os.path.abspath(os.path.normpath(os.path.join(__file__,