Couple fixes for better python3 support and accurate thread handling

Hi there!

Module websocket-client-py3 looks outdated now (buggy socket closing) and websocket-client now support python3 so looks like its a good time to switch to a proper module.

Module _thread usage looks totally pointless - class Connection implements Thread so we can just do connection.start() and do not run thread inside thread for no reason.

Also pong_timer supposed to be cancelled in _stop_timers() as other connection-related timers.