Introduce a gate/check GHA job by webknjaz · Pull Request #97533 · python/cpython

This adds a GHA job that reliably determines if all the required
dependencies have succeeded or not.

It also allows to reduce the list of required branch protection CI
statuses to just one — `check`. This reduces the maintenance burden
by a lot and have been battle-tested across a small bunch of projects
in its action form and in-house implementations of other people.

This action is now in use in aiohttp (and other aio-libs projects),
CherryPy, conda, coveragepy, Open edX, Towncrier some of the Ansible
repositories, pip-tools, all of the jaraco's projects (like
`setuptools`, `importlib_metadata`), some of hynek's projects (like
`attrs`, `structlog`), some PyCQA, PyCA, PyPA and pytest projects, a
few AWS Labs projects. Admittedly, I maintain a few of these but it
seems to address some of the pain folks have:
jaraco/skeleton#55 (comment).

I figured, this might be useful for CPython too, which is why I'm
submitting this patch.

The story behind this is explained in more detail at
https://github.com/marketplace/actions/alls-green#why.

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>