input: Write a function to sum a list of integers provided by the user.
  assert sum_integers([1, 2, 3, 4, 5]) == 15
  assert sum_integers([10, -2, 3, 7, 5]) == 23
  assert sum_integers([0, 0, 0, 0, 0]) == 0
output: def sum_integers(int_list):
      return sum(int_list)