Isn't this bug about the "root of a share" case with ntpath.isabs in 3.x and 2.7 (splitdrive was backported)? For example:

    >>> os.path.isabs("//server/share")
    False
    >>> os.path.splitdrive('//server/share') 
    ('//server/share', '')

vs.

    >>> os.path.isabs('//server/share/')             
    True
    >>> os.path.splitdrive('//server/share/')
    ('//server/share', '/')

I think '//server/share' is an absolute path, and pathlib agrees. Network shares do not maintain a current directory the way drives do (i.e. the hidden environment variable trick). There's no such thing as a share-relative path, nor would there be any way to even write such a path, as compared to "C:drive/relative/path".