[3.13] GH-127078: `url2pathname()`: handle extra slash before UNC drive in URL path (GH-127132) (#127135)

GH-127078: `url2pathname()`: handle extra slash before UNC drive in URL path (GH-127132)

Decode a file URI like `file://///server/share` as a UNC path like
`\\server\share`. This form of file URI is created by software the simply
prepends `file:///` to any absolute Windows path.
(cherry picked from commit 8c98ed846a7d7e50c4cf06f823d94737144dcf6a)

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