Fix DeprecationWarning by avoiding coroutine decorator

Author: pquentin

tests/aiotest/test_callback.py

@@ -45,8 +45,7 @@ async def test_close(self, loop, config):
         await loop.stop().wait()
         loop.close()
 
-        @config.asyncio.coroutine
-        def test():
+        async def test():
             pass
 
         func = lambda: False