[Twisted] On the sneaky caching behavior of twisted.python.filepath.FilePath

Hello, I was reviewing some third-party code today and noticed many bugs in it that arise from FilePath's caching behavior. I found https://github.com/twisted/twisted/issues/3161 (only after I wrote up https://github.com/twisted/twisted/issues/11676 though). The cache behavior is long-standing but I think it needs to go. It represents a dangerous and subtle trap for anyone who tries to use FilePath, which I would *like *to be everyone - but that would be an easier sell without traps like this one. Jean-Paul