GH-125866: Support complete "file:" URLs in urllib (#132378)

Add optional *add_scheme* argument to `urllib.request.pathname2url()`; when
set to true, a complete URL is returned. Likewise add optional
*require_scheme* argument to `url2pathname()`; when set to true, a complete
URL is accepted.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>