Maybe we should review pathlib.py for this kind of issues and first apply the fixes and new tests inside PyPy.  That sounds like a better way to get things done for these rare issues, where CPython is understandably reluctant to do much changes.

Note that the PyPy version of the stdlib already contains fixes that have not been merged back to CPython (or only very slowly), though so far they are the kind of issues that trigger more often on PyPy than on CPython, like GC issues.