Add a send_async method to the Signal

[pgjones]

This allows for signals to send to coroutine receivers by awaiting
them. The _async_wrapper and _sync_wrapper allows for conversion of
sync and async receivers as required if defined. If not defined a
runtime error is raised.

The wrappers are used to avoid any direct tie into asyncio, trio,
greenbacks, asgiref, or other specific async implementation.

[pgjones]

Note that if async coroutines are connected to any signal's receiver_connected or receiver_disconnected there will be a runtime error. I think this is acceptable.

[DonDebonair]

I saw that previous code to supprt async was reverted. Can we merge this one in and get a release soon?

[davidism]

We'll merge this when we're ready and have time. Async as it works in a hybrid environment, and how it will work with Flask and quart, is a very complex question.

[pgjones]

@Secrus what do you make of this?

[Secrus]

This surely needs a proper documentation entry.

[pgjones]

I've added documentation

[Secrus]

Needs a rebase with main, other than that, LGTM.

[djsilcock]

Just as an observation on the principle of least surprise, I would have intuitively expected await some_signal.send_async() to run the handlers concurrently, rather than sequentially - the concurrent behaviour would require something like:

def wrapper(fn):
      @wraps(fn)
      def handler(sender,**kwargs):
          return asyncio.create_task(fn(sender,**kwargs)
      return handler
res=[(rec,await result if asyncio.isfuture(result) else result) for rec,result in some_signal.send(_async_wrapper=wrapper)