Drop pytest-runner and "setup.py test" support

[musicinmybrain]

Removes pytest-runner from setup_requires, and remove test_requires, which is deprecated and only useful with setup.py test.

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.

This is a very small, targeted fix; it would be superseded by any larger build-system modernization like #26 or #23.

[CaselIT]

thanks!