Update pyyaml to 3.13

[pyup-bot]

This PR updates PyYAML from 3.12 to 3.13.

The bot wasn't able to find a changelog for this release. Got an idea?

Links

• PyPI: https://pypi.org/project/pyyaml
• Homepage: http://pyyaml.org/wiki/PyYAML

[codecov]

Codecov Report

Merging #130 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   96.64%   96.64%           
=======================================
  Files           4        4           
  Lines         179      179           
=======================================
  Hits          173      173           
  Misses          6        6

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