Relies on legacy API of importlib.resources

[jaraco]

The project relies on an interface in importlib resources that's marked as legacy and slated for removal. Better would be to rely on the preferred files() API (requires importlib_resources on Python<3.9).

[sigmavirus24]

As mentioned in #147 until importlib.* can actually really once-and-for-all stabilize to a good place with a backport that won't cause us headaches when people use this in an environment with a package with different version constraints on the backport, we won't be upgrading from those deprecated APIs.