Update pytest-runner to 6.0.0

[pyup-bot]

This PR updates pytest-runner from 5.2 to 6.0.0.

Changelog

6.0.0

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* 49: Dropped workaround for older setuptools versions.
* Require Python 3.7.

5.3.2

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* 58: Fixed syntax issue in 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.

Links

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