Significant cleanup x 5.

This commit is the next in a commit chain quietly resurrecting BETSE
for a modern audience and the modern Python ecosystem. Specifically,
this commit:

* Begins unwinding (i.e., permanently removing) a mandatory runtime
  dependency on the deprecated `setuptools.pkg_resources` subpackage.
* Removes `setuptools` altogether as a mandatory runtime dependency from
  our top-level `pyproject.toml` configuration.

(*Ontological ontology in an illogical topology!*)