input: Write a python function to remove keys from a dictionary whose values are greater than a given threshold.
  assert remove_keys({'a': 1, 'b': 2, 'c': 3, 'd': 4}, 2) == {'a': 1, 'b': 2}
  assert remove_keys({'x': 10, 'y': 5, 'z': 7}, 6) == {'y': 5}
  assert remove_keys({'alpha': 3, 'beta': 4, 'gamma': 5}, 4) == {'alpha': 3}
output: def remove_keys(d, threshold):
      return {k: v for k, v in d.items() if v <= threshold}