change chained maps to MutableMapping (#6044)

Fixes #6042

Author: ramalho

stdlib/collections/__init__.pyi

@@ -279,9 +279,9 @@ class defaultdict(Dict[_KT, _VT], Generic[_KT, _VT]):
     def copy(self: _S) -> _S: ...
 
 class ChainMap(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
-    maps: list[Mapping[_KT, _VT]]
-    def __init__(self, *maps: Mapping[_KT, _VT]) -> None: ...
-    def new_child(self: Self, m: Mapping[_KT, _VT] | None = ...) -> Self: ...
+    maps: list[MutableMapping[_KT, _VT]]
+    def __init__(self, *maps: MutableMapping[_KT, _VT]) -> None: ...
+    def new_child(self: Self, m: MutableMapping[_KT, _VT] | None = ...) -> Self: ...
     @property
     def parents(self: Self) -> Self: ...
     def __setitem__(self, k: _KT, v: _VT) -> None: ...