Fix stub return hint consistency with base method (#21)

Author: bzoracler

third_party/3/pandas/core/frame.pyi

@@ -131,7 +131,7 @@ class DataFrame(NDFrame):
     def fillna(self, value: FillValue = ..., method: Optional[FillMethod] = ..., axis: Optional[Axis] = ..., inplace: Optional[bool] = ..., limit: int = ..., downcast: Optional[Dict[Any, Dtype]] = ...) -> Optional[DataFrame]: ...
     def replace(self, to_replace: Optional[ToReplace] = ..., value: Optional[ReplaceValue] = ..., inplace: bool = ..., limit: Optional[int] = ..., regex: bool = ..., method: ReplaceMethod = ...) -> DataFrame: ...
     def shift(self, periods: int = ..., freq: Optional[Frequency] = ..., axis: AxisOption = ..., fill_value: Scalar = ...) -> DataFrame: ...
-    def set_index(self, keys: Union[Label, IndexArray, List[Union[Label, IndexArray]]], drop: bool = ..., append: bool = ..., inplace: bool = ..., verify_integrity: bool = ...) -> DataFrame: ...
+    def set_index(self, keys: Union[Label, IndexArray, List[Union[Label, IndexArray]]], drop: bool = ..., append: bool = ..., inplace: bool = ..., verify_integrity: bool = ...) -> Optional[DataFrame]: ...
     def reset_index(self, level: Optional[Union[Hashable, Sequence[Hashable]]] = ..., drop: bool = ..., inplace: bool = ..., col_level: Hashable = ..., col_fill: Optional[Hashable] = ...) -> Optional[DataFrame]: ...
     def isna(self) -> DataFrame: ...
     def isnull(self) -> DataFrame: ...