* Travis CI: Streamline and use flake8 to find Python syntax errors and undefined names

The __sudo:__ tag is deprecated in Travis CI and __Linux__ and __Python 3.6__ are defaults.

* import numpy as np for line 72

* noqa: Code uses version detection instead of feature detection

https://docs.python.org/3/howto/pyporting.html#use-feature-detection-instead-of-version-detection