Update six to 1.15.0

[pyup-bot]

This PR updates six from 1.13.0 to 1.15.0.

Changelog

1.15.0

------

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

1.14.0

------

- 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.

Links

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

[codecov]

Codecov Report

Merging #141 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #141   +/-   ##
=======================================
  Coverage   30.00%   30.00%           
=======================================
  Files           3        3           
  Lines          10       10           
=======================================
  Hits            3        3           
  Misses          7        7           

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[pyup-bot]

Closing this in favor of #247