[3.12] GH-103631: Fix `PurePosixPath(PureWindowsPath(...))` separator handling (GH-104949) (GH-104991)

For backwards compatibility, accept backslashes as path separators in
`PurePosixPath` if an instance of `PureWindowsPath` is supplied.
This restores behaviour from Python 3.11.

(cherry picked from commit 328422ce6162eb18735a2c0de12f8a696be97d0c)

Co-authored-by: Barney Gale <barney.gale@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>