input: Write a python function to count the number of even numbers in a list.
  assert count_even_numbers([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])==5
  assert count_even_numbers([10, 20, 45, 67, 84, 93])==3
  assert count_even_numbers([5, 7, 9, 8, 6, 4, 3])==3
output: def count_even_numbers(lst):
      return sum(1 for x in lst if x % 2 == 0)