Annotate known magic-method return types (#9131)

stdlib/xml/dom/minidom.pyi

@@ -213,7 +213,7 @@ class CDATASection(Text):
 
 class ReadOnlySequentialNamedNodeMap:
     def __init__(self, seq=...) -> None: ...
-    def __len__(self): ...
+    def __len__(self) -> int: ...
     def getNamedItem(self, name): ...
     def getNamedItemNS(self, namespaceURI: str, localName): ...
     def __getitem__(self, name_or_tuple): ...