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

Tools/Scripts/webkitpy/common/memoized_unittest.py

 # Copyright (c) 2010 Google Inc. All rights reserved.
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-import webkitpy.thirdparty.unittest2 as unittest
+import unittest
 
 from webkitpy.common.memoized import memoized
 
 
 class _TestObject(object):
     def __init__(self):
         self.callCount = 0
 
     @memoized
     def memoized_add(self, argument):
@@ skipping 16 matching lines @@
         # Validate that callCount is working as expected.
         self.assertEqual(test.memoized_add(2), 3)
         self.assertEqual(test.callCount, 2)
 
     def test_tearoff(self):
         test = _TestObject()
         # Make sure that get()/tear-offs work:
         tearoff = test.memoized_add
         self.assertEqual(tearoff(4), 5)
         self.assertEqual(test.callCount, 1)