ringbuf_queue: Add asynchronous get() method.

Author: peterhinch

v3/docs/EVENTS.md

@@ -513,7 +513,9 @@ Synchronous methods (immediate return):
 Asynchronous methods:  
  * `put` Arg: the object to put on the queue. If the queue is full, it will
  block until space is available.
-
+ * `get` Return an object from the queue. If empty, block until an item is
+ available.
+ 
 Retrieving items from the queue:
 
 The `RingbufQueue` is an asynchronous iterator. Results are retrieved using

v3/primitives/ringbuf_queue.py

@@ -64,6 +64,9 @@ def __aiter__(self):
         return self
 
     async def __anext__(self):
+        return await self.get()
+
+    async def get(self):
         while self.empty():  # Empty. May be more than one task waiting on ._evput
             await self._evput.wait()
         r = self._q[self._ri]