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

Tools/Scripts/webkitpy/common/editdistance_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.editdistance import edit_distance
 
 
 class EditDistanceTest(unittest.TestCase):
     def test_edit_distance(self):
         self.assertEqual(edit_distance('', 'aa'), 2)
         self.assertEqual(edit_distance('aa', ''), 2)
         self.assertEqual(edit_distance('a', 'ab'), 1)
         self.assertEqual(edit_distance('ab', 'a'), 1)
         self.assertEqual(edit_distance('ab', 'aa'), 1)
         self.assertEqual(edit_distance('aa', 'ab'), 1)
         self.assertEqual(edit_distance('abd', 'abcdef'), 3)
         self.assertEqual(edit_distance('abcdef', 'abd'), 3)