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