Update pytest to 3.9.1

[pyup-bot]

This PR updates pytest from 3.8.2 to 3.9.1.

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

Links

• PyPI: https://pypi.org/project/pytest
• Changelog: https://pyup.io/changelogs/pytest/
• Homepage: https://docs.pytest.org/en/latest/

[codecov-io]

Codecov Report

Merging #443 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #443   +/-   ##
=======================================
  Coverage   74.93%   74.93%           
=======================================
  Files           5        5           
  Lines         794      794           
  Branches      231      231           
=======================================
  Hits          595      595           
  Misses        146      146           
  Partials       53       53

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