tests/helpers.py: fix py38 vers incompatibility from other PR

tests/helpers.py

 from itertools import product
 import random
-from typing import Any
+from typing import Any, List
 
 import torch
 
 test_dims_rng = random.Random(42)
 
 
-def get_test_dims(min: int, max: int, *, n: int) -> list[int]:
+def get_test_dims(min: int, max: int, *, n: int) -> List[int]:
     return [test_dims_rng.randint(min, max) for _ in range(n)]