The Python SDK uses the package [six](https://github.com/optimizely/python-sdk/blob/fafad4cb53a4f6c3ab8e1ea0beede50e08bcf979/optimizely/decision_service.py#L15) but does not appear to require it in any of the [requirements](https://github.com/optimizely/python-sdk/blob/master/requirements/core.txt)