Add options to configure serial flow control

[jaydenpung]

I encountered an issue when using this library for SIM800. When connecting with putty it works but not with the library. It appears putty default the serial flow control to xonxoff

When we set up the serial, we need to set the xonxoff to true and rtscts, dsrstr to false. This PR change would allow you to use it like this:

from gsmmodem.modem import GsmModem

# set flow control to xonxoff for modem like SIMCOM SIM800
modem = GsmModem(port, 9600, xonxoff=True, rtscts=False, dsrdtr=False)

This is a known issue and had been raised before:
#102
#94

I suspect this repo is no longer maintained. For anyone interested, I have published to pypi and you can use them:

pip install python-gsmmodem-2025

or add to requirements.txt

python-gsmmodem-2025

[zwc456baby]

I'm using an A7670G modem, but I still get a timeout exception: File "D:\software\python38\Lib\site-packages\gsmmodem\modem.py", line 952, in sendSms
raise TimeoutException()
gsmmodem.exceptions.TimeoutException: None
The SMS message seems to be sent, but the program throws an exception.

[zwc456baby]

Sorry, this is my problem. My moderator does not send receipt information in some cases. So it will always block. When I no longer request receipt information, there will be no timeout.