feat(check_for_request): inform number of concurrent pull requests

[mcepl]

Fixes: #4

[jubalh]

Should this not rather be post pull instead of pre push?
This way you might realize that someone already opened a PR for something that you want to do.

[mcepl]

Perhaps both? check_for_request_on_action works for both, but that can be resolved by a simple symlink.

[dmach]

@mcepl I suppose this requires unauthenticated access to the gitea instance, correct?
src.opensuse.org works nicely, but the internal instance errors out with {"message":"Only signed in user is allowed to call APIs."}

I'm thinking if shell scripts are a good idea after all.
Maybe we should keep them to handle the basic functionality of the drop-in directories for the hooks and then use python, which is a prerequisite for osc/git-obs anyway. And we could re-use some of the git-obs functionality incl. the configuration such as URLs, tokens, ssh keys etc.