GH-103631: Fix `PurePosixPath(PureWindowsPath(...))` separator handling (GH-104949)

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

Co-authored-by: Gregory P. Smith <greg@krypto.org>