> The 3.8 backport is much more complicated, as we don't have access to 
> the PathSkipRoot function there. So we can't use the native function.

I guess you missed the comment that I left on the PR a few days ago. The 3.8 backport can use the older PathSkipRootW() in shlwapi.dll [1]. It works similarly, except it doesn't support a drive relative root such as "C:spam" -> ("C:", "spam"), but it's easy enough to handle that case in C. Also, it's documented that it's limited to MAX_PATH, but it works fine with long paths in Windows 10, even if the process does not have long-path support enabled. Anyway, just limit the copy to the first `MAX_PATH - 1` characters. Practically speaking, no root is anywhere near that long. It would require a ridiculously long device name in a "\\.\" device path. 

Examples:

    import ctypes
    shlwapi = ctypes.WinDLL('shlwapi')
    shlwapi.PathSkipRootW.restype = ctypes.c_wchar_p
    path = (ctypes.c_wchar * 1000)()

It returns NULL if there's no root:

    >>> path.value = r'spam'
    >>> shlwapi.PathSkipRootW(path) is None
    True

Drive-relative paths aren't supported the same as they are in PathCchSkipRoot():

    >>> path.value = r'C:spam'
    >>> shlwapi.PathSkipRootW(path) is None
    True

Otherwise it seems to support the same range of paths as PathCchSkipRoot():

    >>> path.value = r'\spam'
    >>> shlwapi.PathSkipRootW(path)
    'spam'

    >>> path.value = r'C:\spam'
    >>> shlwapi.PathSkipRootW(path)
    'spam'

    >>> path.value = r'\\server\share\spam'
    >>> shlwapi.PathSkipRootW(path)
    'spam'

    >>> path.value = r'\\?\C:\spam'
    >>> shlwapi.PathSkipRootW(path)
    'spam'

    >>> path.value = r'\\?\UNC\server\share\spam'
    >>> shlwapi.PathSkipRootW(path)
    'spam'

    >>> path.value = r'\\?\Volume{12345678-1234-1234-1234-123456789ABC}\spam'
    >>> shlwapi.PathSkipRootW(path)
    'spam'

    >>> path.value = r'\\.\PIPE\spam'
    >>> shlwapi.PathSkipRootW(path)
    'spam'

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[1] https://docs.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-pathskiprootw