The workflow also notified on requirements.txt to keep the additional dependencies in sync with the mypy hook, I would probably still include that, but I'm also fine with it fully gone, whatever you prefer. Just wanted to point it out.

Thanks for the catch! Had overlooked that and will revert acoordingly

Additionally, I would probably add a line in Contributing.rst, saying that one can install the pre-commit runs manually and run them on the files if desired. Much like the black line we have, maybe append to that.

Jup, good idea 👍

but wouldn't this just make dependabot not notify us of any new releases for the deleted dev-reqs?

pre-commit.ci takes care of that - see #3085

we can also change the dependabot separator to "-" to conform to our style more.

With the slash, Pycharm automatically groups the branches in a directory style. I like that very much tbh, but in the end it's just a ide-specific gimmick. I'm fine with either way

fwiw, I'm still for automating updating pre-commit's additional_dependencies on dependabot PR's using a custom .yml script, but I don't know if/how that's possible

that would indeed be nice! I found dependabot/dependabot-core#2040 on this - FYI.
having a workflow that triggers on PRs that change reqs.txt and search-replaces in .pre-commit.yml should probably suffice … Not a top priority IMO though :D