Broken out from #22636. Scipy is using cirrus ci for linux aarch64 macos arm64, and musl CI runs. The config file is here, and the CI runs are here. They are happier with it than with travis, which has proven flaky. We will still need travis if we wish to keep the ppc64le CI runs.

Looking at the SciPy PR #17029 I think the steps are

• Start a thread on the mailing list and link it here.
• Sign up for an account and add the app to the repo. Can we join the SciPy one or do we need our own?
• Create some tokens
• Create a workflow
• Set up a cron job for the weekly wheels
• Document it somewhere: we should have some "maintainer tasks" documentation somewhere with a table of all the CI runs and what they do, and how to update dependencies like OpenBLAS and cibuildwheel