Add an alert for python < 2.5.

gclient.py

Index: gclient.py
diff --git a/gclient.py b/gclient.py
index 6817db16940ed7eddd1c2c31e74208fef32fc764..98a81925aad7f69c511a62e30cc751a5ca0ff3d5 100644
--- a/gclient.py
+++ b/gclient.py
@@ -1193,6 +1193,9 @@ def GenUsage(parser, command):
 def Main(argv):
   """Doesn't parse the arguments here, just find the right subcommand to
   execute."""
+  if sys.hexversion < 0x02050000:
+    print >> sys.stderr, (
+        '\nYour python version is unsupported, please upgrade.\n')
   try:
     # Make stdout auto-flush so buildbot doesn't kill us during lengthy
     # operations. Python as a strong tendency to buffer sys.stdout.