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](https://github.com/pypa/setuptools/issues/926) and [pypa/setuptools#510](https://github.com/pypa/setuptools/issues/510).
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