input: Write a function to calculate the sum of the items in a given list.
  assert sum_list([1, 2, 3, 4, 5]) == 15
  assert sum_list([-1, -2, -3, -4, -5]) == -15
  assert sum_list([0, -1, 1, 0]) == 0
  assert sum_list([]) == 0
output: def sum_list(lst):
      return sum(lst)