Swap in `setuptools_scm` to handle package versioning

[Brewster Malevich]

Swap in setuptools_scm to handle package versioning in setup.py. Would just pull package version from git tags so we wouldn't need to manually change version number. Non-released (e.g. dev installs) then have a -dev slug that parts of commit hash and commit distance from last tag - which can be helpful to debug user problems.

Thinking about making this a wider python package guideline for CLI, as it can save us quite a hassle.