warnings.catch_warnings() is only available on python 2.5

tests/presubmit_unittest.py

Index: tests/presubmit_unittest.py
diff --git a/tests/presubmit_unittest.py b/tests/presubmit_unittest.py
index 7f5d57191cbe0fdb953150916445746552226065..32a3829ed0e1a62c6f15c0e25a10b1977b677cb6 100755
--- a/tests/presubmit_unittest.py
+++ b/tests/presubmit_unittest.py
@@ -21,7 +21,11 @@ import presubmit_canned_checks
 class PresubmitTestsBase(unittest.TestCase):
   """Setups and tear downs the mocks but doesn't test anything as-is."""
   def setUp(self):
-    self._warnings_stack = warnings.catch_warnings()
+    if hasattr(warnings, 'catch_warnings'):
+      self._warnings_stack = warnings.catch_warnings()
+    else:
+      self._warnings_stack = None
+    
     warnings.simplefilter("ignore", DeprecationWarning)
     self.original_IsFile = os.path.isfile
     def MockIsFile(f):