Add type annotation for Controller.start() (#420)

aiosmtpd/controller.py

@@ -253,7 +253,7 @@ def _run(self, ready_event: threading.Event) -> None:
         self.loop.close()
         self.server = None
 
-    def start(self):
+    def start(self) -> None:
         """
         Start a thread and run the asyncio event loop in that thread
         """