Replace pkg_resources: deprecated

[petiay]

pkg_resources seems to be used in three places. But it "is deprecated in favor of importlib.resources, importlib.metadata and their backports (importlib_resources, importlib_metadata)": https://setuptools.pypa.io/en/latest/pkg_resources.html

[karllark]

I've heard this one a discussion of another package. Sounds like a straightforward fix. Maybe at the next BEAST HackDay!

[petiay]

Yes, it does look straightforward. At the next Hack day!