Format files with black

- 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

core/gui/__init__.py

@@ -13,4 +13,3 @@ blue, which is supposed to be orange, does the sorting logic, holds selection, e
 
 .. _cross-toolkit: http://www.hardcoded.net/articles/cross-toolkit-software
 """
-