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

Tools/Scripts/webkitpy/layout_tests/views/metered_stream_unittest.py

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+import StringIO
 import logging
 import re
-import StringIO
-import webkitpy.thirdparty.unittest2 as unittest
+import unittest
 
 from webkitpy.layout_tests.views.metered_stream import MeteredStream
 
 
 class RegularTest(unittest.TestCase):
     verbose = False
     isatty = False
 
     def setUp(self):
         self.stream = StringIO.StringIO()
@@ skipping 101 matching lines @@
 
         # The second argument should have a real timestamp and pid, so we just check the format.
         self.assertTrue(re.match('\d\d:\d\d:\d\d.\d\d\d \d+ bar\n', buflist[1]))
 
         self.assertEqual(len(buflist), 2)
 
     def test_log_args(self):
         self.logger.info('foo %s %d', 'bar', 2)
         self.assertEqual(len(self.buflist), 1)
         self.assertTrue(self.buflist[0].endswith('foo bar 2\n'))