Fix asyncio reactor for Python 3.9+

Author: ramikg

cassandra/io/asyncioreactor.py

@@ -173,7 +173,7 @@ def push(self, data):
 
     async def _push_msg(self, chunks):
         # This lock ensures all chunks of a message are sequential in the Queue
-        with await self._write_queue_lock:
+        async with self._write_queue_lock:
             for chunk in chunks:
                 self._write_queue.put_nowait(chunk)