We have so many nice docstrings, but they aren't really rendered
anywhere (expect locally in your IDE), as we did not include the API
docs anywhere.

This change uses sphinx-apidoc to generate autodoc stubs for Sphinx, so
that the docstrings are actually rendered to HTML. This is not perfect
yet (I'm not too happy about the default modules.rst simply listing one
package), but it is good enough for the start and makes it possible to
actually browse the docstrings in a browser.

setup.py is the very old style for packaging, so I wanted to replace it
with something more "modern". pyproject.toml seems like the way to go in
the future.

At first, I wanted to simply configure setuptools using pyproject.toml,
but that support is in beta and seemed to cause some issues with the tox
virtualenvs.

Poetry seems to work fine and provides a better dependency resolver
(given that dependencies are actually specified well) and some other
goodies. For users, nothing much should change, as "pip install" still
works.