Rework setup

[mbakke]

• Move away from setup.py in favor of setup.cfg.
• Use importlib.metadata to set __version__.
• Add PEP517 compliance, so you can do python -m build.
• Fix CI.

[mbakke]

Indeed, the pyproject.toml should work with any PEP517 compliant build frontend and distro tooling.

Now, rumor has it that setup.cfg will go away too eventually, so it may only be a stop-gap...