fixtures: change FixtureDef.cached_result[2] from exception triplet to exception

[bluetech]

I propose changing the type of the 3rd item of FixtureDef.cached_result (which indicates a cached exception outcome of the fixture) from a sys.exc_info style triplet to just the exception instance.

The sys.exc_info exception triplet format is legacy from Python 2 and just complicates things nowadays.

While cached_result is not documented and is a private API, from 680 plugins I have checked out locally, 15 access it. However, none of them care about the type of cached_result[2], except for pytest-yield plugin, but it's already broken in recent pytests and only supports pytest<=4.

[RonnyPfannschmidt]

+1 from me, but id also like to ensure we move the cache to the setup state in future

[bluetech]

+1 from me, but id also like to ensure we move the cache to the setup state in future

I think something like that will be necessary for fixing #11205. There our preferred approach has a single FixtureDef having multiple values at the same time (for different nested scopes), so a single cached_result on the FixtureDef can no longer work.