Require six>=1.9 for Python.

We use six sub-modules that aren't available in old versions. Not specifying
this requirement in the setup will lead to much less readable runtime errors.
It's particuarlly problematic for Mac OSX users because the system comes with
an old six version 1.4.1 by default.

[skip ci]

python/setup.py

@@ -183,7 +183,7 @@ if __name__ == '__main__':
     os.environ['PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION'] = 'cpp'
 
   # Keep this list of dependencies in sync with tox.ini.
-  install_requires = ['six', 'setuptools']
+  install_requires = ['six>=1.9', 'setuptools']
   if sys.version_info <= (2,7):
     install_requires.append('ordereddict')
     install_requires.append('unittest2')

python/tox.ini

@@ -21,6 +21,6 @@ commands =
 # cpp: python setup.py -q test_conformance --cpp_implementation
 deps =
     # Keep this list of dependencies in sync with setup.py.
-    six
+    six>=1.9
     py26: ordereddict
     py26: unittest2