Enable testing merge queues @ GitHub Actions CI/CD

[webknjaz]

This patch allows the project to start using merge queues. They will still have to be enabled in the branch protection settings.

Contributor checklist

• [x] Included tests for the changes.
• [x] PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• [x] Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• [ ] Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[webknjaz]

Hey @jezdez, could you go to the repo's branch protection settings, and enable merge queues there, once this PR is merged?

[jezdez]

@webknjaz done, using squash commits

[jezdez]

[webknjaz]

@webknjaz done, using squash commits

@jezdez please, keep all the options. Squashing is harmful for history.

[jezdez]

@webknjaz It's not a multiple choice config, here are the options:

[webknjaz]

@jezdez then, use merge. It'll make them octopus. Also, I'd drop that checkbox for "only failing" — sometimes, several failing PRs composed will result in a green CI.

[jezdez]

@webknjaz 👍🏻

[webknjaz]

@jezdez thanks! This will help reduce the need for people to "merge main into PR" all the time since the merge queue will take care of checking that several PRs play well together.

[chrysle]

@webknjaz CI seems to have started failing after this pull request. Could you look into this?

[webknjaz]

That's unrelated. The scheduled nightly runs have a slightly different CI matrix which includes testing against the very last commit in the Pip's repository — that's what's failing.