Accept unicode(py2) or bytes(py3)

    Returns:
        py2 type: str
        py3 type: str

def to_string(s, encoding='utf-8'):
    """
    Accept unicode(py2) or bytes(py3)

    Returns:
        py2 type: str
        py3 type: str
    """
    if six.PY2:
        return s.encode(encoding)
    if isinstance(s, bytes):
        return s.decode(encoding)
    return s