PurePosixPath.joinpath() incorrectly handles paths with drives  

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.
