This makes it easy to track all the places that use config.yml, and
hopefully makes things feel cleaner. This also standardizes all places
where config.yml is written out to use UTF-8 as the file encoding.
This also includes a lot of black code format fixes.
This name always confuses me, since there is also test_common.py. And this
module is not actually a test suite, even though it starts with "test".
This also makes for better tab completion, e.g.
python3 -m unittest tests/te[Tab] -> tests/test_
