Remove pytest-runner from build-system.requires (#1659)

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.

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools", "pytest-runner"]
+requires = ["setuptools"]
 build-backend = "setuptools.build_meta"
 
 [project]