input: Write a function to take a positive integer as input and use a while loop to print the numbers counting down from that integer to 1. Print "Done!" after the loop finishes.
  assert countdown(6)==None
  # Expected output: 6 5 4 3 2 1 Done!
  assert countdown(3)==None
  # Expected output: 3 2 1 Done!
  assert countdown(1)==None
  # Expected output: 1 Done!
  assert countdown(10)==None
  # Expected output: 10 9 8 7 6 5 4 3 2 1 Done!
output: def countdown(n):
      while n > 0:
          print(n, end=' ')
          n -= 1
      print("Done!")