close() fails with rtscts=True

[davegravy]

from mpio import Serial

ser = Serial("/dev/ttyS1", 115200, rtscts=True)

ser.write(b"AT\r")
ser.close()

CTS is high preventing the data from being written, and so ser.close() is hanging indefinitely.

Traceback (most recent call last):
  File "mpio-serial-test.py", line 11, in <module>
    ser.close()
  File "/usr/local/lib/python3.8/dist-packages/mpio/serial.py", line 306, in close
    os.close(self._fd)
KeyboardInterrupt
Exception ignored in: <function Serial.__del__ at 0xb6b371d8>
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/mpio/serial.py", line 87, in __del__
  File "/usr/local/lib/python3.8/dist-packages/mpio/serial.py", line 306, in close
OSError: [Errno 9] Bad file descriptor

(doesn't happen with rtscts=False)

Consider adding termios.tcflush(fd, termios.TCOFLUSH) to the close() function