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

Tools/Scripts/webkitpy/layout_tests/port/browser_test_driver_unittest.py

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-import webkitpy.thirdparty.unittest2 as unittest
+import unittest
 
 from webkitpy.common.system.systemhost_mock import MockSystemHost
 
 from webkitpy.layout_tests.port import Port, Driver, DriverOutput
 from webkitpy.layout_tests.port import browser_test, browser_test_driver
 from webkitpy.layout_tests.port.server_process_mock import MockServerProcess
 
 from webkitpy.layout_tests.port.port_testcase import TestWebKitPort
 
 from webkitpy.tool.mocktool import MockOptions
 
 
 class BrowserTestDriverTest(unittest.TestCase):
     def test_read_stdin_path(self):
         port = TestWebKitPort()
         driver = browser_test_driver.BrowserTestDriver(port, 0, pixel_tests=True)
         driver._server_process = MockServerProcess(lines=[
             'StdinPath: /foo/bar', '#EOF'])
         content_block = driver._read_block(0)
         self.assertEqual(content_block.stdin_path, '/foo/bar')
         driver._stdin_directory = None