ERROR: InterpreterNotFound: python3.4

I’m testing this project: and CI fails with python 3.4: ERROR: InterpreterNotFound: python3.4.

As you can see, previous python 3.4 builds completed with success, what is happening here?

The immediate reason is

I don’t know how tox finds interpreters – its documentation glosses over that – so can’t say what exactly is wrong with Xenial from its POV.