Add setuptools_scm

[jeromekelleher]

It would be good to use setuptools_scm to manage versioning for alignment with other tskit-dev repos, as described in the management document: https://github.com/tskit-dev/.github/blob/main/repo_administration.md

It's quite straightforward to set up, and there's no reason it can't coexist with the other version numbers being defined in _version.py.

Was there a particular reason for not using it @petrelharp?

[petrelharp]

Not that I remember, no?