Replace setuptools with importlib_* equivalents
Our version constraint on setuptools was causing installation failures in some environments (e.g. outside a virtualenv), because pip cannot uninstall the "incompatible" system version of setuptools.
But anyway, using setuptools/pkg_resources is deprecated:
https://setuptools.pypa.io/en/stable/pkg_resources.html
Replace our usage of pkg_resources with importlib_resources and importlib_metadata, so that we don't even need setuptools as a runtime dependency anymore.