input: Write a function to filter numbers divisible by seven.
  assert divisible_by_seven([14, 21, 49, 28, 35, 56, 16]) == [14, 21, 49, 28, 35, 56]
  assert divisible_by_seven([7, 14, 21, 20, 35, 40]) == [7, 14, 21, 35]
  assert divisible_by_seven([1, 2, 3, 4, 5, 6]) == []
output: def divisible_by_seven(numbers):
      return [num for num in numbers if num % 7 == 0]