update internals.pyi

Author: jorisvandenbossche

pandas/_libs/internals.pyi

@@ -94,3 +94,7 @@ class BlockValuesRefs:
     def add_reference(self, blk: Block) -> None: ...
     def add_index_reference(self, index: Index) -> None: ...
     def has_reference(self) -> bool: ...
+
+class SetitemMixin:
+    def __setitem__(self, key, value) -> None: ...
+    def __delitem__(self, key) -> None: ...