Add missing return type annotation

Author: natehardison

asyncssh/connection.py

@@ -1202,7 +1202,7 @@ def is_server(self) -> bool:
 
         return self._server
 
-    def is_closed(self):
+    def is_closed(self) -> bool:
         """Return whether the connection is closed"""
 
         return self._close_event.is_set()