Add requirements.txt to require pyserial
Improve serial connection.

Use simpler comport listing from pyserial.

Allow garbage input during connection setup if it still contains the magic
string "SERVOTOR".

Set serOpenDone to True in serHandler. This will allow connection setup to be
slower but still correctly be detected as connected. pyserial provides timeouts
itself, no need to do it twice.

Untested on windows, garbage input changes not applied to windows code.