Missing dependency on setuptools

[ubmarco]

The library uses pkg_resources in multiple locations, e.g. here https://github.com/useblocks/sphinx-collections/blob/c2df8fbd4233670758cfd7c73589e725d955230a/sphinxcontrib/collections/main.py#L2

The library setuptools however does not appear in the direct project dependencies https://github.com/useblocks/sphinx-collections/blob/master/pyproject.toml#L32

This should be added. As an alternative, we can move on to importlib. pkg_resources is deprecated anyway, see https://setuptools.pypa.io/en/latest/pkg_resources.html.