Support earlier versions of Python

[abingham]

Right now we're using 3.6+ APIs in (at least) our use of subprocess.run()'s encoding parameter. We should see if we can support earlier versions, maybe 3.3+.

[abingham]

This might require using something like Popen directly. Maybe it's not worth it. People using this kernel are not likely to be tied to some old version of Python. Let's leave this alone for now and see if anyone squeals.