Bug fix: /dev/ttyUSBx is deleted when upper_tester terminates

            attrs = termios.tcgetattr(self._fd)

            speed = getattr(termios, f"B{self.baud}")

            attrs[4] = attrs[5] = speed

            # Clear HUPCL (hang up on close) and set CLOCAL (ignore modem status lines): without

            # this, closing our fd -- e.g. on Ctrl-C, or just process exit -- drops DTR, which on

            # some USB cellular modules the firmware treats as "host went away" and responds to by

            # detaching/re-enumerating that USB function, i.e. /dev/ttyUSBx disappears even though

            # we never asked for a module reset.

            attrs[2] &= ~termios.HUPCL

            attrs[2] |= termios.CLOCAL

            termios.tcsetattr(self._fd, termios.TCSANOW, attrs)

        except (termios.error, AttributeError) as e:  # e.g. pty

            log.debug("baud rate not set: %s", e)

            log.debug("baud rate/HUPCL/CLOCAL not set: %s", e)

        self._stop_r, self._stop_w = os.pipe()

        self._thread = threading.Thread(target=self._reader, name="modem-reader", daemon=True)

        self._thread.start()