use __future__.with_statement in greentest.py (2.5 compatibility)

greentest/greentest.py

@@ -20,6 +20,7 @@
 # THE SOFTWARE.
 
 # package is named greentest, not test, so it won't be confused with test in stdlib
+from __future__ import with_statement
 import sys
 import unittest
 from unittest import TestCase as BaseTestCase