Fix the “test” extra

[musicinmybrain]

Replace [options.extras_require] with [project.optional-dependencies] as required for pyproject.toml.

Test with:

python3.11 -m venv _e
. _e/bin/activate
pip install -e .[test]

which works as expected after this PR (but not before it).

[mikedh]

Thanks for the fix!