Update to PyTest 9 and native Toml configuration

[PyTest 9](https://docs.pytest.org/en/stable/changelog.html#pytest-9-0-0-2025-11-05) supports native TOML configuration as an alternative to the janky INI-in-TOML syntax.

If we bump our minimum PyTest to 9, we can take advantage of this. Since PyTest isn't an external-impacting dependency, there seems to be no downside here.