async def METHOD_NAME(
    self, status_line: str, headers: "CIMultiDict[str]"
) -> None:
    """Write request/response status and headers."""
    if self._on_headers_sent is not None:
        await self._on_headers_sent(headers)
    # status + headers
    buf = _serialize_headers(status_line, headers)
    self._write(buf)