input: Write a python function to find the median of a list of numbers.
  assert median([1, 2, 3, 4, 5]) == 3
  assert median([1, 3, 3, 6, 7, 8, 9]) == 6
  assert median([1, 2, 3, 4]) == 2.5
output: def median(numbers):
      numbers.sort()
      n = len(numbers)
      middle = n // 2
      
      if n % 2 == 0:
          return (numbers[middle - 1] + numbers[middle]) / 2.0
      else:
          return numbers[middle]