input: Write a function to calculate the mode of a list of numbers.
  assert mode([1, 2, 2, 3, 4, 4, 4, 5]) == [4]
  assert mode([1, 1, 2, 2, 3, 3, 4, 5]) == [1, 2, 3]
  assert mode([10, 20, 20, 30, 30, 30, 40]) == [30]
output: def mode(numbers):
      from collections import Counter
      count = Counter(numbers)
      max_freq = max(count.values())
      return [num for num, freq in count.items() if freq == max_freq]