TYP: define new_leveln as FrozenList or List

Author: Kashif Khan

pandas/core/reshape/reshape.py

@@ -39,6 +39,7 @@
     Index,
     MultiIndex,
 )
+from pandas.core.indexes.frozen import FrozenList
 from pandas.core.series import Series
 from pandas.core.sorting import (
     compress_group_index,
@@ -316,6 +317,9 @@ def get_new_columns(self, value_columns: Index | None):
         stride = len(self.removed_level) + self.lift
         width = len(value_columns)
         propagator = np.repeat(np.arange(width), stride)
+
+        new_levels: FrozenList | list[Index]
+
         if isinstance(value_columns, MultiIndex):
             new_levels = value_columns.levels + (self.removed_level_full,)
             new_names = value_columns.names + (self.removed_name,)