A bot should do it. Because sometimes people throw a tantrum, so it's easier to just ignore a PR. Or a maintainer might post a quick reply, only to be bitten later on. I love and live by open source, but the drama can be exhausting.
Are you really open source in spirit at that point though?
Yes. Both the cathedral and the bazaar are valid approaches to OSS project management, with pros and cons.
For those curious apart from myself:
Cathedral -> centralized control
Bazaar -> decentralized control
"open source" doesn't mean "accepting drive-by contributions from unknown authors". "open source" means "open source". If you want to apply your patches, you're free to fork the repo.