Speed up imports by using importlib instead of pkg_resources

Speed up imports by up to a second by replacing uses of pkg_resources with the new Python standard library module importlib.resources (or, for Python < 3.7, the backport importlib_resources). The old pkg_resources module is known to be slow because it does a lot of work on startup.

See, for example, pypa/setuptools#926 and pypa/setuptools#510.