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.
I was curious about the spread of use. And I've just discovered that
it is now in use in aiohttp (and other aio-libs projects), CherryPy,
some of the Ansible repositories, all of the jaraco's projects (like
`setuptools`, `importlib_metadata`), some PyCQA, PyCA 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:
https://github.com/jaraco/skeleton/pull/55#issuecomment-1106638475.
The story behind this is explained in more detail at
https://github.com/marketplace/actions/alls-green#why.
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