Move deployment to Github Actions

[nikolai-b]

We are using CircleCI to deploy but now Github Actions exist we should you this instead. More robust and less dependances.
This would mean adding the private deploy SSH key. Probably best to make a new one.

[Robinlovelace]

That would be good. Another advantage: GitHub Actions are commonly used for testing and auto-building package websites and I have some understanding of them whereas CircleCI always seemed a bit opaque. Great plan!