File from apptools.io should implement os.PathLike

[corranwebster]

If we were to add a __fspath__ method and register the class with os.PathLike (where available) it would mean that File objects could be used in a similar way to pathlib.Path objects throughout Python.

As a secondary consideration, using a pathlib path object instead of a string as the source of truth for the File class would simplify a lot of the logic; it would probably make sense to also duplicate the pathlib.Path API (alongside the current API).