Update six to 1.13.0

[pyup-bot]

This PR updates six from 1.12.0 to 1.13.0.

Changelog

1.13.0

------

- 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

[codecov-io]

Codecov Report

Merging #901 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
- Coverage   94.15%   94.02%   -0.13%     
==========================================
  Files          10       10              
  Lines         787      787              
==========================================
- Hits          741      740       -1     
- Misses         46       47       +1

Impacted Files	Coverage Δ
etcd3/watch.py	85.81% <0%> (-0.71%)	⬇️

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[pyup-bot]

Closing this in favor of #1004