Run flake8 on CI

[dbnicholson]

black only does formatting while flake8 is a linter, so it's good to run both. In order to get there:

• Fix a failing test (I can split this out if preferred)
• Fix a recent black diff (this is also in #414)
• Fix all the existing flake8 warnings and errors
• Install and run flake8 from the formatting workflow job