input: Write a python function to keep only even numbers from a list.
  assert keep_even_only([2, 5, 8, 9, 10, 13, 11]) == [2, 8, 10]
  assert keep_even_only([1, 3, 5, 7]) == []
  assert keep_even_only([4, 6, 8, 10]) == [4, 6, 8, 10]
  assert keep_even_only([0, 12, 15, 22, 27]) == [0, 12, 22]
output: def keep_even_only(lst):
      return [num for num in lst if num % 2 == 0]