Annotate known magic-method return types (#9131)

stubs/boto/boto/exception.pyi

@@ -21,7 +21,7 @@ class BotoServerError(StandardError):
     box_usage: Any
     def __init__(self, status, reason, body: Any | None = ..., *args) -> None: ...
     def __getattr__(self, name): ...
-    def __setattr__(self, name, value): ...
+    def __setattr__(self, name, value) -> None: ...
     def startElement(self, name, attrs, connection): ...
     def endElement(self, name, value, connection): ...