Add explicit return types for frame processor methods (#546)

Author: lukasIO

livekit-rtc/livekit/rtc/frame_processor.py

@@ -22,12 +22,12 @@ def _on_stream_info_updated(
         room_name: str,
         participant_identity: str,
         publication_sid: str,
-    ): ...
+    ) -> None: ...
 
-    def _on_credentials_updated(self, *, token: str, url: str): ...
+    def _on_credentials_updated(self, *, token: str, url: str) -> None: ...
 
     @abstractmethod
     def _process(self, frame: T) -> T: ...
 
     @abstractmethod
-    def _close(self): ...
+    def _close(self) -> None: ...