Use importlib over __file__ or runtime dependencies on setuptools

[effigies]

Use of __file__ makes a package unzippable. setuptools' pkg_resources permits packages to access data files whether zipped or unzipped. It turns out, however, that runtime dependencies on setuptools are considered bad form. Python 3.7 has introduced importlib.resources, and there's a backport importlib-resources.

Migration guide: https://importlib-resources.readthedocs.io/en/latest/migration.html

[effigies]

Done in #1190.