Support `pytest.tool` in pyproject.toml

Since version 9.0, Pytest supports using TOML syntax instead of the old ini compatibility syntax (see https://docs.pytest.org/en/stable/changelog.html and https://docs.pytest.org/en/stable/reference/customize.html#pytest-toml)

I have confirmed locally that it works, so maybe its nice to add it to the documentation. I wouldn't mind opening a PR myself.