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?

[alyx-db]

@musicinmybrain

Thanks for your submission! I'll pass this along to the team for review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.57%. Comparing base (b848f4b) to head (adeb64b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
- Coverage   65.12%   64.57%   -0.55%     
==========================================
  Files          31       31              
  Lines       54897    54897              
  Branches     3902     3902              
==========================================
- Hits        35749    35452     -297     
- Misses      18989    19411     +422     
+ Partials      159       34     -125     

Flag	Coverage Δ
integration	?
unit	64.57% <ø> (ø)

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