PurePosixPath.joinpath() incorrectly handles paths with drives

[mrbean-bremen]

This is a spin-off of #1067, where a respective test had been added by @lengau.
Happens only with Python >= 3.12 and under Windows.

In the real filessystem we get:

>>> PurePosixPath("C:").joinpath("/Program Files")
PurePosixPath('/Program Files')

while in the fake filesystem this produces PurePosixPath('C:/Program Files') instead.