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

Tools/Scripts/webkitpy/tool/grammar_unittest.py

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-import webkitpy.thirdparty.unittest2 as unittest
+import unittest
 
 from webkitpy.tool.grammar import join_with_separators
 
 class GrammarTest(unittest.TestCase):
 
     def test_join_with_separators(self):
         self.assertEqual(join_with_separators(["one"]), "one")
         self.assertEqual(join_with_separators(["one", "two"]), "one and two")
         self.assertEqual(join_with_separators(["one", "two", "three"]), "one, two, and three")