Really, this is not meant to be set by the user in the config. But if they
add something harmless that'll be ignored anyway, it seems that throwing an
error is too much. So only throw the error if it is set wrongly.
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_
