Fix TypeError in slcan.py when formatting standard (non-extended) CAN…

[andrejrcarvalho]

Try to fix a type error on the _send_frame function of the slcan.py driver.
I found this bug while using the Jupyter console of the Dronecan GUI app:

After some tinkering with the pydronecan module, I've found that the marker variable was being put in the wrong place when we try to send a standard frame (11 bit identifier) in the following function of the TxWorker:

def _send_frame(self, frame):
        marker = 'D' if frame.canfd else 'T'
        dlc_len = CANFrame.datalength_to_dlc(len(frame.data))
        line = '%s%X%s\r' % (('%c%08X' if frame.extended else 't%03X') % (marker, frame.id),
                             dlc_len,
                             binascii.b2a_hex(frame.data).decode('ascii'))

        self._conn.write(line.encode('ascii'))
        self._conn.flush()