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