input: Write a python function to find the smallest number in a list.
  assert smallest_number([3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]) == 1
  assert smallest_number([10, 20, 30, 5, 40, 50]) == 5
  assert smallest_number([-10, -20, -30, -5, -40, -50]) == -50
output: def smallest_number(numbers):
      return min(numbers)