Replaces use of __file__

[Ben-Mack]

The __file__ attribute is not guaranteed to exist, and causes problems for a number of packaging solutions. Please see https://github.com/indygreg/PyOxidizer/issues/69

The guide recommend using importlib.resources: https://pyoxidizer.readthedocs.io/en/latest/oxidized_importer_resource_files.html#porting-code-to-modern-resources-apis