Update six to 1.12.0

[pyup-bot]

This PR updates six from 1.10.0 to 1.12.0.

Changelog

1.12.0

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- 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`.

1.11.0

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- Pull request 178: `with_metaclass` now properly proxies `__prepare__` to the
underlying metaclass.

- Pull request 191: Allow `with_metaclass` to work with metaclasses implemented
in C.

- Pull request 203: Add parse_http_list and parse_keqv_list to moved
urllib.request.

- Pull request 172 and issue 171: Add unquote_to_bytes to moved urllib.parse.

- Pull request 167: Add `six.moves.getoutput`.

- Pull request 80: Add `six.moves.urllib_parse.splitvalue`.

- Pull request 75: Add `six.moves.email_mime_image`.

- Pull request 72: Avoid creating reference cycles through tracebacks in
`reraise`.

Links

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