[3.13] gh-85110: Preserve relative path in URL without netloc in urllib.parse.urlunsplit() (GH-123179) (#123187)

gh-85110: Preserve relative path in URL without netloc in urllib.parse.urlunsplit() (GH-123179)
(cherry picked from commit 90c892efeaae28bd849a01b42842f19dcd67b9f4)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>