[3.13] GH-126766: `url2pathname()`: handle 'localhost' authority (GH-127129) (#127130)

GH-126766: `url2pathname()`: handle 'localhost' authority (GH-127129)

Discard any 'localhost' authority from the beginning of a `file:` URI. As a
result, file URIs like `//localhost/etc/hosts` are correctly decoded as
`/etc/hosts`.
(cherry picked from commit ebf564a1d3e2e81b9846535114e481d6096443d2)

Co-authored-by: Barney Gale <barney.gale@gmail.com>