Update six to 1.13.0

[pyup-bot]

This PR updates six from 1.12.0 to 1.13.0.

Changelog

1.13.0

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- Issue 298, pull request 299: Add `six.moves.dbm_ndbm`.

- Issue 155: Add `six.moves.collections_abc`, which aliases the `collections`
module on Python 2-3.2 and the `collections.abc` on Python 3.3 and greater.

- Pull request 304: Re-add distutils fallback in `setup.py`.

- Pull request 305: On Python 3.7, `with_metaclass` supports classes using PEP
560 features.

Links

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