input: Write a python function to calculate the sum of all numbers in a list.
  assert sum_list([1, 2, 3, 4, 5]) == 15
  assert sum_list([10, 20, 30, 40]) == 100
  assert sum_list([5, -5, 10, -10, 15]) == 15
output: def sum_list(numbers):
      return sum(numbers)