input: Write a python function to find the median of a list of numbers.
  assert median([1, 3, 5, 7]) == 4.0
  assert median([1, 2, 3, 4, 5]) == 3
  assert median([10, 20, 30]) == 20
  assert median([5, 2, 9, 1, 5, 6]) == 5.0
output: def median(numbers):
      numbers.sort()
      n = len(numbers)
      mid = n // 2
      
      if n % 2 == 0:
          return (numbers[mid - 1] + numbers[mid]) / 2.0
      else:
          return numbers[mid]