input: Write a python function to sum even numbers in a list.
  assert sum_evens([1, 2, 3, 4, 5, 6, 7, 8, 9]) == 20
  assert sum_evens([10, 21, 32, 43, 54, 65]) == 96
  assert sum_evens([4, 4, 4, 4, 4]) == 20
output: def sum_evens(numbers):
      return sum(num for num in numbers if num % 2 == 0)