Poetry does the right thing by default, so no more need to declare this.

This is used for the README and the CHANGELOG, as they are read during
setup.py. If we do not include them, building a source distribution will
fail.

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.