Remove support of Python2.5 and fix py3 warnings

purchase.py

@@ -1,6 +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 __future__ import with_statement
 import datetime
 import copy
 from decimal import Decimal

setup.py

@@ -50,7 +50,6 @@ setup(name='trytond_purchase',
         'Natural Language :: German',
         'Natural Language :: Spanish',
         'Operating System :: OS Independent',
-        'Programming Language :: Python :: 2.5',
         'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Topic :: Office/Business',