Py3: Fix ToASCII method
    
    in Py2 to_text method returns Py2 non-unicode string, but in Py3 to_text method
    returns Py3 default (unicode) string. So only in Py2 we have to decode
    str to unicode.
    
    https://fedorahosted.org/freeipa/ticket/5935
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>