Use poetry for the dev environment

[lazka]

This adds a pyproject.toml, which in itself enables isolated builds in
pip, but we don't have any build deps anyway.

This allows specifying dev dependencies and "poetry install" to install them,
which makes hacking on mutagen easier.

Also cleans up CI a bit by using poetry there too, cache wheels, and
build the docs in the same job.

sdist/wheel is still done with setuptools, so no change there.