make client compatible with both python2 and python3
Many experiments are starting to test python 3 and need a jobsub_client that supports this.
We need to continue to support python2 for some time as well. There are libraries (six) and a body of knowledge dedicated to writing python that runs correctly under both language branches, see https://python-future.org/compatible_idioms.html
Move as much of the client code as possible to dual compatibility