Update serial_comms.py

[havefunworkhard]

Seems like self.serial.read(1) in serial_comms.py returns bytes in Python 3.x.
But code expects strings and compares returned value with strings like RX_EOL_SEQ = '\r\n' or ''. The comparison result will always be False.
In this case SerialComms will never _handleLineRead and TimeoutException will be raised.

[alejandroMA]

I been waiting 3 moths for a pull request to be merge or rejected :(

[babca]

@foXes68 Let's make the project alive again.
I've merged all the fixes including yours here:
https://github.com/babca/python-gsmmodem

please check it out and do a pull request there if you found any bugs.