added list[Levels] as per pandas-dev#489

Author: muddi900

pandas-stubs/core/frame.pyi

@@ -997,7 +997,9 @@ class DataFrame(NDFrame, OpsMixin):
         margins_name: _str = ...,
         observed: _bool = ...,
     ) -> DataFrame: ...
-    def stack(self, level: Level = ..., dropna: _bool = ...) -> DataFrame | Series: ...
+    def stack(
+        self, level: Level | list[Level] = ..., dropna: _bool = ...
+    ) -> DataFrame | Series: ...
     def explode(self, column: _str | tuple, ignore_index: _bool = ...) -> DataFrame: ...
     def unstack(
         self,