input: Write a python function to sum all positive numbers in a list.
  assert sum_numbers([1, 2, 3, 4, 5]) == 15
  assert sum_numbers([-1, 2, -3, 4, -5]) == 6
  assert sum_numbers([0, -1, -2, -3, -4]) == 0
output: def sum_numbers(lst):
      return sum(x for x in lst if x > 0)