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-io]

Codecov Report

Merging #193 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #193   +/-   ##
=======================================
  Coverage   37.33%   37.33%           
=======================================
  Files          30       30           
  Lines        2882     2882           
=======================================
  Hits         1076     1076           
  Misses       1806     1806

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