Replace six with the current version which passes checklicenses.py.

third_party/six/PKG-INFO

-Metadata-Version: 1.1
-Name: six
-Version: 1.9.0
-Summary: Python 2 and 3 compatibility utilities
-Home-page: http://pypi.python.org/pypi/six/
-Author: Benjamin Peterson
-Author-email: benjamin@python.org
-License: MIT
-Description: Six is a Python 2 and 3 compatibility library.  It provides utility functions
-        for smoothing over the differences between the Python versions with the goal of
-        writing Python code that is compatible on both Python versions.  See the
-        documentation for more information on what is provided.
-        
-        Six supports every Python version since 2.5.  It is contained in only one Python
-        file, so it can be easily copied into your project. (The copyright and license
-        notice must be retained.)
-        
-        Online documentation is at http://pythonhosted.org/six/.
-        
-        Bugs can be reported to https://bitbucket.org/gutworth/six.  The code can also
-        be found there.
-        
-        For questions about six or porting in general, email the python-porting mailing
-        list: http://mail.python.org/mailman/listinfo/python-porting
-        
-Platform: UNKNOWN
-Classifier: Programming Language :: Python :: 2
-Classifier: Programming Language :: Python :: 3
-Classifier: Intended Audience :: Developers
-Classifier: License :: OSI Approved :: MIT License
-Classifier: Topic :: Software Development :: Libraries
-Classifier: Topic :: Utilities