Self-hosted alternative to Recaptcha

Gitlab can use Recaptcha to fight bots, but this introduces dependency on a 3rd-party service.

It would be nice if Gitlab had a self-hosted alternative so that we can repel bots and don't depend on Google at the same time.

The self-hosted spam protection does not have to be very complex. Maybe a honeypot input field that needs to stay empty and a little piece of Javascript that adds a required hidden field will suffice.