Remove thirdparty/unittest2 from webkitpy and require python2.7.

Tools/Scripts/webkitpy/common/thread/messagepump_unittest.py

 # Copyright (C) 2010 Google Inc. All rights reserved.
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 # modification, are permitted provided that the following conditions are
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 #    * Redistributions of source code must retain the above copyright
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-import webkitpy.thirdparty.unittest2 as unittest
+import unittest
 
 from webkitpy.common.thread.messagepump import MessagePump, MessagePumpDelegate
 from webkitpy.common.thread.threadedmessagequeue import ThreadedMessageQueue
 
 
 class TestDelegate(MessagePumpDelegate):
     def __init__(self):
         self.log = []
 
     def schedule(self, interval, callback):
@@ skipping 34 matching lines @@
         self.assertEqual(delegate.log, [
             'schedule',
             'schedule',
             'message_available: Hello',
             'message_available: There',
             'schedule',
             'message_available: More',
             'message_available: Messages',
             'final_message_delivered'
         ])