input: Write a python function to calculate the sum of even numbers in a list.
  assert even_sum([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 30
  assert even_sum([10, 21, 32, 43, 54]) == 96
  assert even_sum([11, 13, 15, 17, 19]) == 0
output: def even_sum(lst):
      return sum(num for num in lst if num % 2 == 0)