Black will automatically fix a lot of the exceptions we previously allowed for
directives, so we don't need them in our custom `flake8_formatter` anymore.
- [x] remove `E501` (long line) exceptions for directives from `flake8_formatter`,
as they won't help us now.
- [x] Refine exceptions for long URLs in the `flake8_formatter`.
- [x] Adjust the mock `flake8-package` to exhibit the exceptions we still allow.
- [x] Update style tests for new `flake8-package`.
- [x] Blacken style test.
549ba1ed32