Coerce *s* to `str`.

    To keep six with lower version, see Issue 4169, we copy this function
    from six == 1.12.0.

    TODO(yuhguo): remove this function when six >= 1.12.0.

    For Python 2:
      - `unicode` -> encoded to `str`
      - `str` -> `str`

    For Python 3:
      - `str` -> `str`
      - `bytes` -> decoded to `str`

def ensure_str(s, encoding="utf-8", errors="strict"):
    """Coerce *s* to `str`.

    To keep six with lower version, see Issue 4169, we copy this function
    from six == 1.12.0.

    TODO(yuhguo): remove this function when six >= 1.12.0.

    For Python 2:
      - `unicode` -> encoded to `str`
      - `str` -> `str`

    For Python 3:
      - `str` -> `str`
      - `bytes` -> decoded to `str`
    """
    if six.PY3:
        text_type = str
        binary_type = bytes
    else:
        text_type = unicode  # noqa: F821
        binary_type = str
    if not isinstance(s, (text_type, binary_type)):
        raise TypeError("not expecting type '%s'" % type(s))
    if six.PY2 and isinstance(s, text_type):
        s = s.encode(encoding, errors)
    elif six.PY3 and isinstance(s, binary_type):
        s = s.decode(encoding, errors)
    return s