[3.12] GH-126766: `url2pathname()`: handle empty authority section. (GH-126767) (#126837)

GH-126766: `url2pathname()`: handle empty authority section. (GH-126767)

Discard two leading slashes from the beginning of a `file:` URI if they
introduce an empty authority section. As a result, file URIs like
`///etc/hosts` are correctly parsed as `/etc/hosts`.
(cherry picked from commit cae9d9d20f61cdbde0765efa340b6b596c31b67f)

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