Add annotation for collections.ChainMap.__missing__(key) (#4784)

This method has been present in the CPython implementation since its introduction in 3.3.
https://github.com/python/cpython/blob/v3.3.0/Lib/collections/__init__.py\#L783

stdlib/3/collections/__init__.pyi

@@ -321,3 +321,4 @@ class ChainMap(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     def __getitem__(self, k: _KT) -> _VT: ...
     def __iter__(self) -> Iterator[_KT]: ...
     def __len__(self) -> int: ...
+    def __missing__(self, key: _KT) -> _VT: ...  # undocumented