improve RTS/DTR control

[camelator]

Ampy does not connect to MaixPi board with Ubuntu 18.04
As explain here:
dhylands/rshell#91
this is related to the RTS / DTR settings in the constructor of class Serial.
adding these two lines in file pyboard.py correct the bug (like the miniterm source code).
self.serial.rts=False
self.serial.dtr=False
Unfortunately I am not a python developper, maybe someone can add some command line parameter ?

[curiouswala]

I don't have a MaixPi board to test this and it's not officially supported.