Drop support for Path as argument of iter_compatible_reader (/writer?)

[Carreau]

Following the current flow in napari/npe2 it looks to me like iter_compat_reader (at least, maybe write as well) will never get a Path, it will only get strings.

It's not like the hook spec themselves, and though I would be in favor of also making them stricter, this is more of an internal api.

Another reason is that napari uses:

for url in event.mimeData().urls():
            if url.isLocalFile():
                filenames.append(url.toLocalFile())
            else:
                filenames.append(url.toString())

Which in then ends ends up in iter_compat_reader so my guess is that some of the things are not representable by path anyway.

[Carreau]

(note I'm also slightly confused as to why the logic differ that much between drop files on the viewer, and using one of File->open)