StreamWriter.drain is a coroutine (#880)

See https://docs.python.org/3/library/asyncio-stream.html#asyncio.StreamWriter.drain

stdlib/3.4/asyncio/streams.pyi

@@ -83,6 +83,7 @@ class StreamWriter:
     def can_write_eof(self) -> bool: ...
     def close(self) -> None: ...
     def get_extra_info(self, name: str, default: Any = ...) -> Any: ...
+    @coroutines.coroutine
     def drain(self) -> None: ...
 
 class StreamReader: