Maybe it's helpful to warn about `try` / `except` around `open_nursery`?

I'm not sure this is a common mistake, but now that strict exception groups are a thing try/except around either trio.open_nursery or trio.run is never going to happen. (except maybe cancellation? I haven't thought through that part)

Stuff like:

async def main():
  try:
    async with trio.open_nursery() as nursery:
      ...
  except KeyboardInterrupt:
    print("...")
    raise

(maybe worth prototyping and seeing if it catches anything?)

Start by prototyping the shown try/except KeyboardInterrupt pattern around trio.open_nursery and checking how strict exception groups behave. Resolve the open question about cancellation and whether this is a common mistake; done means the warning scope and prototype outcome are decided.