GH-107465: Add `pathlib.Path.from_uri()` classmethod. (#107640)

This method supports file URIs (including variants) as described in RFC 8089, such as URIs generated by `pathlib.Path.as_uri()` and `urllib.request.pathname2url()`.

The method is added to `Path` rather than `PurePath` because it uses `os.fsdecode()`, and so its results vary from system to system. I intend to deprecate `PurePath.as_uri()` and move it to `Path` for the same reason.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>