Added automerge

[max-sixty]

What is your issue?

@pydata/xarray

Because our pipeline takes a while, it can be helpful to have an option to "merge when tests pass" — I've now set that up. So you can click here and it'll do just that.

Someone annoyingly / confusingly, the "required checks" need to be specified manually, in https://github.com/pydata/xarray/settings/branch_protection_rules/2465574 — there's no option for just "all checks".

So if we change the checks — e.g. add Python 3.11 — that list needs to be updated. If we remove a check from the our CI and don't update the list, it won't be possible to merge the PR without clicking the red "Admin Override" box — so we should keep it up to date.

[max-sixty]

Once it's clicked it looks like:

[TomNicholas]

Oh that's awesome!