Automating release

[jakirkham]

To simplify the release process, wonder if it would be worthwhile to bake most of the release steps into a GHA workflow that responds to a git tag

It could for example generate release notes that are included in a GitHub release. Also it could create the whl & sdist artifacts and upload them to PyPI

The conda-forge bot already submits new releases. Though we could start them more quickly using the @conda-forge-admin, please update version bot request in a dask-image feedstock issue