Using `AnyPath` for type hinting

[jdcc-betterup]

I'd like to type annotate with AnyPath to show that a function could accept local or cloud paths, but my type checker yells because AnyPath doesn't have an __fspath__ (or isn't compatible with ReadBuffer). This would be simple to add, but there might be other undesirable follow-on effects from making it path-like that I'm not aware of (specifically thinking of #128).

[pjbull]

Unfortunately, I believe that it will not work to type things with AnyPath because it dynamically resolves things to two classes that have no formal type relationship. Maybe when PathBase lands and is backported this could be possible. Open to implementations if they could work, but I believe we had looked at this without a good resolution in #146 (we can keep discussion there).

That said I think that typing with union like Path | CloudPath should work or typing with PathLike.

[rpmcginty]

@pjbull fwiw, it seems pretty straightforward to implement the PathBase #517