bump_dev_version is Py3-only

[dwhswenson]

This may not be a real problems, but bump_dev_version requires Python 3 (due to from json import JSONDecodeError). In general, that script is only run during a deployment cycle, which means that the Python to use in deployment currently needs to be Python 3.

Still, as long as Autorelease technically supports Python 2, it would probably make sense for that part of Autorelease to also support Python 2.