Feature/websocket

[csgpwi]

Add support for jenkins websockets and python3 to slave.py

[foxylion]

Hi @csgpwi, did you test your changes?
Since when do the paths have changed in Jenkins?
Will this break existing users of the image?

[pjwiseman]

@foxylion

1. I ran adhoc tests of the slave.py changes against python2 and python3, in a fresh venv, and with/without the SLAVE_WEBSOCKET env to test backward compatibility.
2. Slave terminology was deprecated in Jenkins 2.0. See https://issues.jenkins.io/browse/JENKINS-42816 for the main epic. The slave-agent to jenkins-agent change was released 2.264 (2020-10-27). I made the change out of concern that the old name would eventually be removed.
3. The slave terminology change will break existing implementations running a jenkins version prior to 2.264. I'm happy to revert that part of the change if you think it best. I don't have the capacity to implement support for both old and new names during the transition period until jenkins remove support for the old naming.