scripts: twister: Fix serial_py referenced before assignment

            flash_with_test = dut.get('flash_with_test')

            if flash_with_test is None:

                flash_with_test = self.options.device_flash_with_test

            serial_pty = dut.get('serial_pty')

            flash_before = dut.get('flash_before')

            if flash_before is None:

                flash_before = self.options.flash_before and (not (flash_with_test or serial_pty))

            id = dut.get('id')

            runner = dut.get('runner')

            runner_params = dut.get('runner_params')

            serial_pty = dut.get('serial_pty')

            serial = dut.get('serial')

            baud = dut.get('baud', None)

            product = dut.get('product')