input: Write a function to filter numbers not divisible by three.
  assert filter_by_three([2, 4, 7, 8, 10, 12, 15, 20]) == [2, 4, 7, 8, 10, 20]
  assert filter_by_three([3, 6, 9, 12, 15]) == []
  assert filter_by_three([1, 2, 4, 5, 7, 8]) == [1, 2, 4, 5, 7, 8]
output: def filter_by_three(numbers):
      return [num for num in numbers if num % 3 != 0]