Automatic package versioning

[tomwhite]

Probably following this guide: https://learn.scientific-python.org/development/guides/packaging-classic/#git-tags-official-pypa-method

[TomNicholas]

@maxjones kindly helped me get this all set up for VirtualiZarr - you could probably use that as a minimal example of how to get the same level of packaging automation in Cubed.