Turns out it somehow breaks our `to_asyncio` error relay since obvi
`asyncio`'s runtime seems to specially handle it (prolly via
`isinstance()` ?) and it caused our
`test_aio_cancelled_from_aio_causes_trio_cancelled()` to hang..
Further, obvi `unpack_error()` won't be able to find the type def if not
kept inside `._exceptions`..
So given all that, revert the change/move as well as:
- tweak the aio-from-aio cancel test to timeout.
- do `trio.sleep()` conc with any bg aio task by moving out nursery
block.
- add a `send_sigint_to: str` parameter to
`test_sigint_closes_lifetime_stack()` such that we test the SIGINT
being relayed to just the parent or the child.
4f1db1ff52