Migrate pkg_resources to importlib

[guyer]

pkg_resources is deprecated, but importlib.resources requires Python 3.7 and importlib.metadata requires Python 3.8.

Fixes #944

[guyer]

Important

pkg_resources still used in fipy.tests.test().
It's unclear what the modern equivalent is of EntryPoint.parse() and, anyway, all of this raises

WARNING: Testing via this command is deprecated and will be removed in a future version. Users looking for a generic test entry point independent of test runner are encouraged to use tox.