TYP: Fix core/groupby/generic.py lexsort attr-defined type ignore

GH37715

Author: Mateusz

pandas/core/groupby/generic.py

@@ -26,6 +26,7 @@
 
 import numpy as np
 
+from pandas._libs import Interval
 from pandas._libs import reduction as libreduction
 from pandas._typing import (
     ArrayLike,
@@ -650,12 +651,9 @@ def value_counts(
 
         if is_interval_dtype(lab.dtype):
             # TODO: should we do this inside II?
+            lab_interval = cast(Interval, lab)
 
-            # error: "ndarray" has no attribute "left"
-            # error: "ndarray" has no attribute "right"
-            sorter = np.lexsort(
-                (lab.left, lab.right, ids)  # type: ignore[attr-defined]
-            )
+            sorter = np.lexsort((lab_interval.left, lab_interval.right, ids))
         else:
             sorter = np.lexsort((lab, ids))