I was only able to reproduce this by installing six v1.6.1 which is explicitly unsupported by keystone (which requires >= v1.7.0 in requirements.txt). The latest version of six is v1.8.0:
https://pypi.python.org/pypi/six
Can you confirm the version of six that you're running?
$ python -c "import six; print(six.__version__)"
I was only able to reproduce this by installing six v1.6.1 which is explicitly unsupported by keystone (which requires >= v1.7.0 in requirements.txt). The latest version of six is v1.8.0:
https:/ /pypi.python. org/pypi/ six
Can you confirm the version of six that you're running?
$ python -c "import six; print(six. __version_ _)"