GH-123599: `url2pathname()`: handle authority section in file URL (#126844)

In `urllib.request.url2pathname()`, if the authority resolves to the
current host, discard it. If an authority is present but resolves somewhere
else, then on Windows we return a UNC path (as before), and on other
platforms we raise `URLError`.

Affects `pathlib.Path.from_uri()` in the same way.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>