Detect TaskStatus not being used #471

I think flake8-async could probably detect that something with a TaskStatus parameter can potentially not call it... of course the rule would be a bit wrong, but I think being overly strict is fine? (e.g. no passing into helper functions, no usage in a loop, no unreachability checks)

See also python-trio/trio#3454. I wanted to do this via types but for backwards compatibility reasons that's not possible.