Update six to 1.15.0

[pyup-bot]

This PR updates six from 1.12.0 to 1.15.0.

Changelog

1.15.0

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- Pull request 331: Optimize `six.ensure_str` and `six.ensure_binary`.

1.14.0

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- Issue 288, pull request 289: Add `six.assertNotRegex`.

- Issue 317: `six.moves._dummy_thread` now points to the `_thread` module on
Python 3.9+. Python 3.7 and later requires threading and deprecated the
`_dummy_thread` module.

- Issue 308, pull request 314: Remove support for Python 2.6 and Python 3.2.

- Issue 250, issue 165, pull request 251: `six.wraps` now ignores missing
attributes. This follows the Python 3.2+ standard library behavior.

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