Remove pytest-runner / setup.py test support

[musicinmybrain]

Removes pytest-runner from setup_requires, and removes 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.

Fixes #401.

This PR is similar to dropbox/stone#354.

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• SDK Code Change
• Example/Test Code Change

Validation

• Does tox pass? (no new regressions, anyway)
• Do the tests pass?