python3 support

[nhoffman]

I'll mainly target python 3.8 for easy integration with ubuntu 20.4, but assume we'll get 3.9 compatibility as well.

[nhoffman]

@sminot - heads up that I'm hoping to work on this soon - are there any considerations given your use for refset creation (if you are still using it)? I'll want to update the Dockerfile as well, and see that you are identified as the maintainer.

[sminot]

Is this something that would be useful for you, @jgolob?