atcommander: force out of bootload mode, Python3 compatible

[mikeclement]

The radio occasionally gets stuck in bootloader mode. This patch replicates a solution from ardupilot (libraries/GCS_MAVLink/GCS_Common.cpp) to unstick the radio. The solution is incorporated into the --force option, making it more assertive than before.

This patch also slightly adjusts some of the radio response timeouts to make the code more responsive overall.

Finally, Michael Day added Python3 compatibility.