input: Write a python function to find the closest pair of numbers in a list.
  assert find_closest_pair([1, 3, 4, 8, 13, 17, 20]) == (3, 4)
  assert find_closest_pair([10, 22, 28, 29, 40]) == (28, 29)
  assert find_closest_pair([7, 14, 9, 3, 11]) == (9, 11)
output: def find_closest_pair(numbers):
      numbers.sort()
      closest_pair = (numbers[0], numbers[1])
      min_diff = abs(numbers[1] - numbers[0])
      
      for i in range(1, len(numbers) - 1):
          diff = abs(numbers[i + 1] - numbers[i])
          if diff < min_diff:
              min_diff = diff
              closest_pair = (numbers[i], numbers[i + 1])
      
      return closest_pair