tests: remove pytest-runner / setup.py test support

[musicinmybrain]

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Description

Please describe briefly your pull request.

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 (since the PR updates tox.ini) with tox.

This would supersede #184.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases. N/A, this affects launching the tests
• I've added relevant documentation. N/A, using setup.py test was not documented
• I've marked translation strings. N/A, no string changes
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions). N/A, <15 lines and lacking creative work
• I've NOT included third-party code (copy/pasted source code or new dependencies).
  • If you have added third-party code (copy/pasted or new dependencies), please reach out to an architect on Discord.

Frontend

• I've followed the CSS/JS and React guidelines. N/A
• I've followed the web accessibility guidelines. N/A
• I've followed the user interface guidelines. N/A

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