GH-85168: Use filesystem encoding when converting to/from file URIs

[barneygale]

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.

• Issue: urllib should fsdecode percent-encoded parts of file URIs on Unix #85168

[barneygale]

The freedesktop file URI spec is a useful reference here:

There is no specified encoding for filenames, and although we hope
that eventually all filenames will be encoded in UTF8 we can't rely on
this, because then we would be unable to e.g. rename a file with a
misencoded filename.

Also:

Some current apps generate file URIs by converting the filename from
whatever locale the application runs in to UTF8. This behavior means
that URIs can't be converted to filenames without knowing the
locale of the application that produced them, and that not all valid
filenames can be converted to URIs. Such behavior is not allowed,
and should be changed.

[serhiy-storchaka]

LGTM in general. But we should support URIs containing not quoted non-ASCII characters.

[barneygale]

Thanks for the review - I've made the changes you suggested

[serhiy-storchaka]

LGTM.

You can also use fsdecode(TESTFN_UNDECODABLE) and TESTFN_UNENCODABLE in some tests if you wish.