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
 from trytond.model import ModelView, ModelSQL, fields
 from trytond.transaction import Transaction
 from trytond.pool import Pool

setup.py

@@ -49,7 +49,6 @@ setup(name='trytond_purchase_invoice_line_standalone',
         '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',