Use GitHub's automerge instead of Mergify

[hugovk]

We first set up Mergify in 2020 to automerge PRs once certain conditions were met: CI has passed, PR is approved, automerge label added: #4931.

Since then, GitHub has added native automerge which is a bit nicer to use: no need to add a label, and while the CI is still running, the "Merge pull request" button becomes an "Enable auto-merge" button, which will do the merge when the CI passes.

Docs: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/automatically-merging-a-pull-request

I've created a ruleset for the main branch at 🔒 https://github.com/python-pillow/Pillow/settings/rules/7135275. There are lots of settings, but I've mirrored the Mergify ones:

Require these status checks to pass:

• Docker Test Successful
• Lint
• MinGW
• Test Successful
• Windows Test Successful
• (Cygwin omitted as it'll be removed in Drop support for Python 3.9 #9119)

Also left this enabled, because we don't want to accidentally delete main:

• Restrict deletions
  Only allow users with bypass permissions to delete matching refs.

We don't need to add a label, because it's now signalled by clicking "Enable auto-merge".

And we don't require explicit approval, because that's also implicit when clicking "Enable auto-merge" (similarly, for a PR where the CI has already passed, we don't require explicit approvals).

If for some reason a status check fails but we still want to merge (maybe some dependency is temporarily failing to download because a server is down), as admins we can still click to bypass the rules and merge.

Screenshots of all the settings

[hugovk]

If for some reason a status check fails but we still want to merge (maybe some dependency is temporarily failing to download because a server is down), as admins we can still click to bypass the rules and merge.

Updated ruleset for this:

[hugovk]

Also requires "Allow auto-merge" at https://github.com/python-pillow/Pillow/settings. I also checked "Automatically delete head branches" to keep things tidy:

[radarhere]

#9097 and #8926 are showing an interesting side effect of these changes in settings - when we skip workflows, because we're only building docs or only building wheels, the 'Successful' checks are now just showing as 'pending'.

It doesn't cause any problems, it makes you think something might be wrong.

[hugovk]

Hmm, it actually does cause a problem, because it blocks merge (although you can use admin permissions to bypass it):

Let's hold off on this for now, and I've disabled the ruleset ("Enforcement status: Disabled") at https://github.com/python-pillow/Pillow/settings/rules/7135275.