input: Write a function to countdown from a given positive integer to 1 using a while loop. The function should take a single argument, the positive integer, and print each number sequentially until it counts down to 1, followed by "Done!".
  assert countdown(6)==None
  assert countdown(3)==None
  assert countdown(1)==None
  
  def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
output: def countdown(num):
      while num > 0:
          print(num)
          num -= 1
      print("Done!")