Mark various dunder methods as positional-only in the stdlib (#14528)

stdlib/array.pyi

@@ -58,6 +58,7 @@ class array(MutableSequence[_T]):
     def tounicode(self) -> str: ...
 
     __hash__: ClassVar[None]  # type: ignore[assignment]
+    def __contains__(self, value: object, /) -> bool: ...
     def __len__(self) -> int: ...
     @overload
     def __getitem__(self, key: SupportsIndex, /) -> _T: ...