gh-89760: Fix os.path.realpath() for volume GUID paths on Windows (GH-155396)

The \\?\ prefix was stripped from the resolved path if it could not be
resolved without the prefix and failed with the same error as the original
path.  This produced an invalid, seemingly relative path for a junction
which points to a volume without a drive letter.  The prefix is now only
stripped for drive-letter and UNC paths.