support python 3.6+ by changing to ensure_future.

src/announce_server/server.py

@@ -137,10 +137,18 @@ def start_server(address, port, heartbeat_interval, heartbeat_timeout):
         The timeout for waiting for a response in seconds.
     """
     loop = asyncio.get_event_loop()
-    heartbeat_task = loop.create_task(
+    # Python 3.7+, coroutines only:
+    # heartbeat_task = loop.create_task(
+    #     heartbeat(sio, heartbeat_interval, heartbeat_timeout)
+    # )
+    # aiohttp_app = loop.create_task(web._run_app(app, host=address, port=port))
+    
+    # Python 3.6+ compatible. Supports any awaitable:
+    heartbeat_task = asyncio.ensure_future(
         heartbeat(sio, heartbeat_interval, heartbeat_timeout)
     )
-    aiohttp_app = loop.create_task(web._run_app(app, host=address, port=port))
+
+    aiohttp_app = asyncio.ensure_future(web._run_app(app, host=address, port=port))
 
     exit_handler = create_exit_handler(loop, heartbeat_task)
     signal.signal(signal.SIGINT, exit_handler)