input: Write a function to return a list of numbers divisible by seven.
  assert divisible_by_seven([14, 21, 22, 35, 49]) == [14, 21, 35, 49]
  assert divisible_by_seven([1, 2, 3, 4, 5, 6]) == []
  assert divisible_by_seven([7, 14, 28, 42, 56]) == [7, 14, 28, 42, 56]
output: def divisible_by_seven(numbers):
      return [num for num in numbers if num % 7 == 0]