Migrate from pkg_resources to importlib or backports

[stevepiercy]

From https://setuptools.pypa.io/en/latest/pkg_resources.html

Caution

Use of pkg_resources is deprecated in favor of importlib.resources, importlib.metadata and their backports (importlib_resources, importlib_metadata). Some useful APIs are also provided by packaging (e.g. requirements and version parsing). Users should refrain from new usage of pkg_resources and should work to port to importlib-based solutions.

It might be good to drop EOLed Python versions at the same time.

Whenever this issue is resolved, then I can remove the setuptools requirement in this PR: plone/plone.restapi#1815.