Fix return type of asyncio.StreamReader.exception

Author: rijenkii

stdlib/asyncio/streams.pyi

@@ -106,7 +106,7 @@ class StreamWriter:
 
 class StreamReader:
     def __init__(self, limit: int = 65536, loop: events.AbstractEventLoop | None = None) -> None: ...
-    def exception(self) -> Exception: ...
+    def exception(self) -> Exception | None: ...
     def set_exception(self, exc: Exception) -> None: ...
     def set_transport(self, transport: transports.BaseTransport) -> None: ...
     def feed_eof(self) -> None: ...