Add collections.ChainMap.__bool__ (#7391)

Author: AlexWaygood

stdlib/collections/__init__.pyi

@@ -380,6 +380,7 @@ class ChainMap(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     def __len__(self) -> int: ...
     def __contains__(self, key: object) -> bool: ...
     def __missing__(self, key: _KT) -> _VT: ...  # undocumented
+    def __bool__(self) -> bool: ...
     def setdefault(self, key: _KT, default: _VT = ...) -> _VT: ...
     @overload
     def pop(self, key: _KT) -> _VT: ...