input: Write a function to count positive numbers in a list.
  assert count_positive_numbers([1, -2, 3, -4, 5, 6]) == 4
  assert count_positive_numbers([-1, -2, -3, -4]) == 0
  assert count_positive_numbers([0, 1, 2, 3, 4]) == 4
output: def count_positive_numbers(lst):
      return sum(1 for x in lst if x > 0)