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.

This moves the updater scripts into a subfolder, which keeps them
separated better from the rest of the package. In addition, we now have
the "fietsupdate" command instead of using "python -m
fietsboek.updater".

The other code kinda assumed that we can turn the resource path into a
package by replacing the slashes (path separators) with dots. That
turned out to not really work in the end, especially if the resource
subfolders don't have a __init__.py in there.

This uses the .files() API (available in Python 3.9 and backported in
importlib_resources) to better handle folders in the resources.