comm: compat fix for pyserial 2.7 vs 3.4
comm: compat fix for pyserial 2.7 vs 3.4

Closes #2468

Jaesin/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint,foosel/OctoPrint,Jaesin/OctoPrint,foosel/OctoPrint