Automate versioning

[elinscott]

Currently our workflow for keeping track of version numbers is quite hands-on. Perhaps it would be good to automate this more

e.g. following the example of optimade, where they use this github action to watch for releases, and if they are semver they trigger a workflow that updates the version number in various files
(via update_docs.sh -> invoke setver calls the function setver from here)

In this configuration the releases become the single point of truth