Update six to 1.12.0

[pyup-bot]

This PR updates six from 1.11.0 to 1.12.0.

Changelog

1.12.0

------

- Issue 259, pull request 260: `six.add_metaclass` now preserves
`__qualname__` from the original class.

- Pull request 204: Add `six.ensure_binary`, `six.ensure_text`, and
`six.ensure_str`.

Links

• PyPI: https://pypi.org/project/six
• Changelog: https://pyup.io/changelogs/six/
• Repo: https://github.com/benjaminp/six
• Docs: https://pythonhosted.org/six/