Remove pytest-runner from setup_requires

[musicinmybrain]

ISSUE TYPE

• Bugfix

SUMMARY

Removes pytest-runner from setup_requires.

The pytest-runner package has been deprecated upstream for some time, and the project is now archived: https://github.com/pytest-dev/pytest-runner/tree/v6.0.1?tab=readme-ov-file#deprecation-notice

Furthermore, the "setup.py test" command was removed in setuptools 72: https://github.com/pypa/setuptools/blob/v75.8.0/NEWS.rst#v7200

This does not affect running the tests with pytest or tox.

Fixes #57.

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[musicinmybrain]

As a follow-up, I have replaced tests_require – which is specific to the old setup.py test – with a new test extra.