Python 3.12.7 (main, Oct  8 2024, 00:39:14) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pathlib; pathlib.PureWindowsPath("C:\\A") / pathlib.PurePosixPath("/foo/C:\\B").relative_to("/")
PureWindowsPath('C:/B')

Python 3.13.0rc3 (main, Oct  7 2024, 23:18:57) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pathlib; pathlib.PureWindowsPath("C:\\A") / pathlib.PurePosixPath("/foo/C:\\B").relative_to("/")
PureWindowsPath('C:/A/foo/C:/B')