Build up the MIME payload for the POST data

def _build_multipart(cls, data):
        """
        Build up the MIME payload for the POST data
        """
        boundary = b'--------------GHSKFJDLGDS7543FJKLFHRE75642756743254'
        sep_boundary = b'\n--' + boundary
        end_boundary = sep_boundary + b'--'
        end_items = end_boundary, b"\n",
        builder = functools.partial(
            cls._build_part,
            sep_boundary=sep_boundary,
        )
        part_groups = map(builder, data.items())
        parts = itertools.chain.from_iterable(part_groups)
        body_items = itertools.chain(parts, end_items)
        content_type = 'multipart/form-data; boundary=%s' % boundary.decode('ascii')
        return b''.join(body_items), content_type