Add a warning if the current version of python is not 2.5

PRESUBMIT.py

 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Top-level presubmit script for depot tools.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into depot_tools.
 """
 
 
 def CommonChecks(input_api, output_api):
   results = []
+  import sys
+  if not sys.version.startswith('2.5'):
+    # Depot_tools has the particularity that it needs to be tested on python
+    # 2.5. But we don't want the presubmit check to fail if it is not installed.
+    results.append(output_api.PresubmitNotifyResult(
+        'You should install python 2.5 and run ln -s $(which python2.5) python.'
+        '\n'
+        'A great place to put this symlink is in depot_tools.\n'
+        'Otherwise, you break depot_tools on python 2.5, you get to keep the '
+        'pieces.'))
+
 
   results.extend(input_api.canned_checks.CheckOwners(input_api, output_api))
   black_list = list(input_api.DEFAULT_BLACK_LIST) + [
       r'^cpplint\.py$',
       r'^tests[\/\\]\w+?[\/\\].+']
   results.extend(input_api.canned_checks.RunPylint(
       input_api,
       output_api,
       white_list=[r'.*\.py$', r'^git-try$'],
       black_list=black_list))
@@ skipping 69 matching lines @@
   return CommonChecks(input_api, output_api)
 
 
 def CheckChangeOnCommit(input_api, output_api):
   output = []
   output.extend(CommonChecks(input_api, output_api))
   output.extend(input_api.canned_checks.CheckDoNotSubmit(
       input_api,
       output_api))
   return output