input: Write a python function to sum a list of numbers.
  assert sum_list([1, 2, 3, 4, 5]) == 15
  assert sum_list([10, 20, 30, 40, 50]) == 150
  assert sum_list([-1, -2, -3, -4, -5]) == -15
output: def sum_list(numbers):
      return sum(numbers)