- Format all files with black
- Update tox.ini flake8 arguments to be compatible
- Add black to requirements-extra.txt
- Reduce ignored flake8 rules and fix a few violations
It's now `DupeGuru` that holds it and passes it to `get_dupe_groups()`,
the only place where it's actually used in `Scanner`.
This will make the SE/ME/PE merge easier by allowing us to instantiate
the Scanner on-the-fly since it doesn't hold state anymore.