Add type annotations

[jaraco]

It would be valuable for projects downstream to have type annotations in order to validate interfaces and pass type checks.

Please consider this a feature request to add type annotations and the py.typed marker.

[emckean]

Thanks @jaraco! We appreciate the suggestion -- since we're a small nonprofit with limited engineering resources, I'm not sure when this will get on our roadmap, but we'd be happy to look at a pull request!