Travis CI: Add Python 3.8 to the testing (#1401)

* Travis CI: Add Python 3.8 to the testing

Py38 generates RuntimeWarnings...

$ __codespell__
```
/home/travis/virtualenv/python3.8.0/lib/python3.8/codecs.py:905:
RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode,
    the default buffer size will be used
file = builtins.open(filename, mode, buffering)
```

* Fix Python >= 3.8 RuntimeWarning for file buffering

* placate flake8

* Use a backslash despite PEP8 to placate flake8

* FIX: Warnings

Co-authored-by: Eric Larson <larson.eric.d@gmail.com>