File from apptools.io should implement os.PathLike

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).