Enforce python 2.6 or later.

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index c24fdb6afc47926c51765b472da152e3b0018de4..003b6e43a17a2fb0c6b59d5caa3e2623fd770b8a 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1387,6 +1387,11 @@ def GenUsage(parser, command):
 def main(argv):
   """Doesn't parse the arguments here, just find the right subcommand to
   execute."""
+  if sys.hexversion < 0x02060000:
+    print >> sys.stderr, (
+        '\nYour python version %s is unsupported, please upgrade.\n' %
+        sys.version.split(' ', 1)[0])
+    return 2
   # Reload settings.
   global settings
   settings = Settings()