Bug 1460470 - Make run-task Python 3.5+ only; r=mshal

A try push converting run-task to Python 3 seemed to complete without
error.

Since it is annoying writing code that needs to work on both Python
2 and 3, let's require Python 3 and remove code for supporting Python 2.

We implement a version check enforcing Python 3.5+. This is because
we're supposed to be standardizing on 3.5+ everywhere. I want to
prevent accidental usage of older Python 3 versions.

MozReview-Commit-ID: 4vATLZ6Si2e