Switch to setuptools-scm / hatch-vcs for version management

[maresb]

As per @jessegrabowski's complaint, the version.txt is clunky, and we should have it done automatically via git tags.

Also note to self, update the build part of the PyPI workflow to make it simpler.

Please feel free to pester me about this.

[williambdean]

Is this part of the release workflow?

[maresb]

Ya