Initialize _session member on YouTube controller (#925)

* Initialize `_session` member on YouTube controller

* Fix mypy lint warnings

pychromecast/controllers/youtube.py

@@ -84,6 +84,7 @@ def __init__(self, timeout: float = 10) -> None:
         super().__init__(YOUTUBE_NAMESPACE, APP_YOUTUBE)
         self.status_update_event = threading.Event()
         self._timeout = timeout
+        self._session = None
 
     def start_session_if_none(self) -> None:
         """