Remove support of Python2.5 and fix py3 warnings

opportunity.py

@@ -1,7 +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.
 "Sales extension for managing leads and opportunities"
-from __future__ import with_statement
 import datetime
 import time
 from trytond.model import ModelView, ModelSQL, ModelWorkflow, fields

setup.py

@@ -48,7 +48,6 @@ setup(name='trytond_sale_opportunity',
         'Natural Language :: French',
         'Natural Language :: German',
         'Operating System :: OS Independent',
-        'Programming Language :: Python :: 2.5',
         'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Topic :: Office/Business',