ignore ChainedUndefined.__getitem__ return-type annotation

* base impl/assignment is improperly annotated, but it was already ignored

src/jinja2/runtime.py

@@ -989,7 +989,7 @@ def __getattr__(self, name: str) -> "ChainableUndefined":
 
         return self
 
-    def __getitem__(self, _: str) -> "ChainableUndefined":
+    def __getitem__(self, _: str) -> "ChainableUndefined":  # type: ignore
         return self