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]

1 files changed, 1 insertions, 1 deletions

diff --git a/python/setup.py b/python/setup.py
index 1e71a702..2a5513b7 100755
--- a/python/setup.py
+++ b/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')