Fixes #263.

Alternative to and closes #259.

At the moment the 3.12-dev builds are failing, causing every build to show as failed.

It's failing because greenlet doesn't yet support 3.12. The tracking issue is at python-greenlet/greenlet#323 and there's a PR at python-greenlet/greenlet#327 but it's not ready yet.

In the meantime, let's skip the two tests when greenlet isn't available.

Also bump GitHub Actions, allow building feature branches, and we can fold the PyPy config into the main list. workflow_dispatch adds a button to the UI to be able to trigger new builds, occasionally useful.

And remove pypy3.7, the newest PyPy only supports 3.8 and 3.9:

• https://www.pypy.org/posts/2022/12/pypy-v7311-release.html