Replace `pkg_resources` with `importlib.resources`

[RobertoRoos]

I saw a warning here at some point. Also note https://setuptools.pypa.io/en/latest/pkg_resources.html

Maybe only with newer versions of Python?

[RobertoRoos]

@windelbouwman could you try to run the tests locally again? There are new instructions in the readme in master and I cleaned up the requirements some.
CI is passing in #19 , so not sure it's just a Python version thing.