[3.13] GH-120423: `pathname2url()`: handle forward slashes in Windows paths (GH-126593) (#126764)

GH-120423: `pathname2url()`: handle forward slashes in Windows paths (GH-126593)

Adjust `urllib.request.pathname2url()` so that forward slashes in Windows
paths are handled identically to backward slashes.
(cherry picked from commit bf224bd7cef5d24eaff35945ebe7ffe14df7710f)

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