Use extend-exclude for flake8 and black

[apaleyes]

This PR makes a bit more extensive use of extend-exclude option, pun intended.

I've hit a couple of issues running all format checks because in my dev setup .venv lives in the project root. So both tools would validated files inside this .venv.

Black by default can respect .gitignore file, which covers .venv, .tox and many more. But if you use exclude option, this default behavior is overriden. So for black we simply use extend-exclude instead of exclude, and then clean up in other places.

flake8 doesn't respect .gitignore at all, but it has sensbile defaults that cover .tox. Thus for flake8 we only need to add .venv to the list of excludes, with the same extend-exclude option.

While this is wasn't an issue in Trieste development so far, I argue it would very contributor-friendly to support .venv in project root, as this setup is quite popular.

[hstojic]

looks reasonable to me, thanks for spotting this!

[hstojic]

ready for merging in?

[apaleyes]

Thanks, @hstojic ! I can't merge it in myself though