`_stdin_hook_default` incorrectly uses `msg_ready`

[BrenBarn]

See this line. msg_ready is now async, so its return value can't be treated as a boolean anymore. I suppose it needs to be wrapped in run_sync.

[davidbrochart]

Thanks for reporting @BrenBarn. Indeed, this cannot work. Rather than wrapping in run_sync, we could make _stdin_hook_default an async function, and support both async and sync stdin_hook. What do you think?

[BrenBarn]

Having support for either sync or async stdin_hook would be great. I don't have much of an opinion on whether _stdin_hook_default should be sync or async, although I suppose making it async makes sense as it is called by an async function and calls async functions itself.