Thanks for the report and for the PR. Could you give us a little bit more information about your use case? Couldn't you make the class you want to use implement the __fspath__ protocol?

    import pathlib as p

    class Spam:
        def __fspath__(self):
            return 'pathlib.py'

And we can use it like:

    >>> p.Path('Lib') / Spam()
    PosixPath('Lib/pathlib.py')
    >>> (p.Path('Lib') / Spam()).exists()
    True

(3.4 and 3.5 are in security-fix-only mode, so I removed them from the versions field.)