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

Tools/Scripts/webkitpy/style/checkers/png_unittest.py

 # Copyright (C) 2012 Balazs Ankes (bank@inf.u-szeged.hu) University of Szeged
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions
 # are met:
 # 1.  Redistributions of source code must retain the above copyright
 #     notice, this list of conditions and the following disclaimer.
 # 2.  Redistributions in binary form must reproduce the above copyright
 #     notice, this list of conditions and the following disclaimer in the
 #     documentation and/or other materials provided with the distribution.
 #
 # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 # A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 # OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 # SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 # LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 # DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 # THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 """Unit test for png.py."""
 
-import webkitpy.thirdparty.unittest2 as unittest
+import unittest
+
 from png import PNGChecker
 from webkitpy.common.system.filesystem_mock import MockFileSystem
 from webkitpy.common.system.systemhost_mock import MockSystemHost
 
 
 class MockSCMDetector(object):
 
     def __init__(self, scm, prop=None):
         self._scm = scm
         self._prop = prop
@@ skipping 85 matching lines @@
         self.assertEqual(len(errors), 1)
 
         file_path = "foo-expected.png"
         fs.write_binary_file(file_path, "Dummy binary data")
         scm = MockSCMDetector('git')
         errors = []
         checker = PNGChecker(file_path, mock_handle_style_error, scm, MockSystemHost(os_name='linux', filesystem=fs))
         checker.check()
         self.assertEqual(len(errors), 2)
         self.assertEqual(errors[0], (0, 'image/png', 5, 'Image lacks a checksum. Generate pngs using run-webkit-tests to ensure they have a checksum.'))