Use minimum python version (3.4) for integration tests

We depend on python 3.4, however if people add code to BuildStream which accidentally uses features from > 3.4, these tend to go unnoticed until it blows up in the user's face.

We aught to always be testing against the minimum required version of python in our integration tests.

Note that this might mean that we should have separate docker images for testing purposes and for the bst-here convenience script - it might make sense to use something more recent for the convenience script than we use for the integration tests (maybe more recent versions of python have bugs fixed or are more optimized).