input: Write a function to find the smallest number in a list.
  assert find_smallest([12, 5, 9, 8, 10]) == 5
  assert find_smallest([3, 2, 1, 4]) == 1
  assert find_smallest([6, 7, 8, 5, 9]) == 5
output: def find_smallest(numbers):
      if not numbers:
          return None
      smallest = numbers[0]
      for num in numbers[1:]:
          if num < smallest:
              smallest = num
      return smallest