Update pytest-runner to 5.3.1

[pyup-bot]

This PR updates pytest-runner from 5.1 to 5.3.1.

Changelog

5.3.1

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* Refreshed package metadata.

5.3.0

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* Require Python 3.6 or later.
* Refreshed package metadata.

5.2

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* 50: This project is deprecated.

Links

• PyPI: https://pypi.org/project/pytest-runner
• Changelog: https://pyup.io/changelogs/pytest-runner/
• Repo: https://github.com/pytest-dev/pytest-runner/

[pyup-bot]

Closing this in favor of #447