input: Write a function to count the number of positive numbers in a list.
  assert count_positive([-2, 3, 4, -7, 6]) == 3
  assert count_positive([1, -1, 2, -2, 3, -3, 4, -4]) == 4
  assert count_positive([-10, -20, -30, 0, 10, 20, 30, 40]) == 4
output: def count_positive(lst):
      return sum(1 for x in lst if x > 0)