input: Write a python function to count even numbers in a list.
  assert count_even_numbers([1, 2, 3, 4, 5, 6, 7, 8])==4
  assert count_even_numbers([10, 21, 32, 43, 54, 65, 76])==4
  assert count_even_numbers([1, 3, 5, 7, 9])==0
output: def count_even_numbers(lst):
      return sum(1 for x in lst if x % 2 == 0)