input: Write a python function to count even numbers in a list.
  assert count_even([1, 2, 3, 4, 5, 6]) == 3
  assert count_even([10, 15, 20, 25, 30]) == 3
  assert count_even([11, 13, 17, 19]) == 0
output: def count_even(numbers):
      return sum(1 for num in numbers if num % 2 == 0)