Log Message:
py-test-runner: update to 6.0.0.

v6.0.0
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* #49: Dropped workaround for older setuptools versions.
* Require Python 3.7.

v5.3.2
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* #58: Fixed syntax issue in changelog.

v5.3.1
======

* Refreshed package metadata.

v5.3.0
======

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