[style/quality] Moving to isort 5.0.0 + style/quality on datasets and metrics
Ready for review @lhoestq, just updated a few 156 files here
Move the repo to isort 5.0.0.

Also start testing style/quality on datasets and metrics.

Specific rule: we allow F401 (unused imports) in metrics to be able to add imports to detect early on missing dependencies.
Maybe we could add this in datasets but while cleaning this I've seen many example of really unused imports in dataset so maybe it's better to have it as a line-by-line nova instead of a general rule like in metrics.