actually fixup typing

Author: MarcoGorelli

pandas/core/frame.py

@@ -6160,6 +6160,8 @@ def f(vals) -> tuple[np.ndarray, int]:
             and subset in self.columns
         ):
             subset_iterable = (subset,)
+        else:
+            subset_iterable = cast(Iterable, subset)
 
         # Verify all columns in subset exist in the queried dataframe
         # Otherwise, raise a KeyError, same as if you try to __getitem__ with a