use Contextmanagers to handle StopIteration in generators

[RonnyPfannschmidt]

• prepare example test for stopiteration passover issue
• WIP: use contextmanagers instead of yield from

yield from is a generator boundary that transfers StopIteration into a RuntimeError

closes #12929

[graingert]

src/_pytest/threadexception.py and src/_pytest/unraisableexception.py both use regular trylast hooks instead of generators, so the conflicts can be resolved by checking them out from main

[graingert]

this won't pass without a new Pluggy release

[nicoddemus]

@RonnyPfannschmidt what is the status here? Do you plan to make a pluggy release yourself?

[RonnyPfannschmidt]

Indeed

I intend to give this priority by the end of the month