Use setuptools_scm instead of hard-coding version in const.py

[bigjools]

It's impossible to do a release from the gitub releases page as the build version is hard-coded in the spinach/const.py file, which is picked up by setup.py.

We can use setuptools_scm instead which will get the version from the git tag.

While we're at it, convert to a modern pyproject.toml and use Hatch to build.