With version 3.4, StackStorm code itself will only run on Python 3. For the v3.4 release, we have chosen to run on Python 3.6 across all of our supported platforms. For users still on Ubuntu 16.04, you will need to source your own Python 3.6 packages, but we have been using the Python 3.6 Ubuntu PPA without many issues. Looking forward to StackStorm 3.5, we will be removing the ability to install Python 2 packs.