[3.12] GH-125069: Fix inconsistent joining in `WindowsPath(PosixPath(...))` (GH-12515...

[3.12] GH-125069: Fix inconsistent joining in `WindowsPath(PosixPath(...))` (GH-125156) (#125410)

`PurePath.__init__()` incorrectly uses the `_raw_paths` of a given
`PurePath` object with a different flavour, even though the procedure to
join path segments can differ between flavours.

This change makes the `_raw_paths`-enabled deferred joining apply _only_
when the path flavours match.

(cherry picked from commit cb8e5995d89d9b90e83cf43310ec50e177484e70)

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