[3.12] GH-126212: Fix removal of slashes in file URIs on Windows (GH-126214) (#126591)

GH-126212: Fix removal of slashes in file URIs on Windows (GH-126214)

Adjust `urllib.request.pathname2url()` and `url2pathname()` so that they
don't remove slashes from Windows DOS drive paths and URLs. There was no
basis for this behaviour, and it conflicts with how UNC and POSIX paths are
handled.
(cherry picked from commit 54c63a32d06cb5f07a66245c375eac7d7efb964a)

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