[3.12] GH-85168: Use filesystem encoding when converting to/from `file` URIs (GH-126852) (#127040)

GH-85168: Use filesystem encoding when converting to/from `file` URIs (GH-126852)

Adjust `urllib.request.url2pathname()` and `pathname2url()` to use the
filesystem encoding when quoting and unquoting file URIs, rather than
forcing use of UTF-8.

No changes are needed in the `nturl2path` module because Windows always
uses UTF-8, per PEP 529.
(cherry picked from commit c9b399fbdb01584dcfff0d7f6ad484644ff269c3)

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