input: Write a python function to simulate a rock-paper-scissors game and determine the winner.
  assert rock_paper_scissors('rock', 'scissors')=='You win!'
  assert rock_paper_scissors('paper', 'rock')=='You win!'
  assert rock_paper_scissors('scissors', 'paper')=='You win!'
  assert rock_paper_scissors('rock', 'rock')=="It's a draw!"
  assert rock_paper_scissors('paper', 'scissors')=='You lose!'
  assert rock_paper_scissors('scissors', 'rock')=='You lose!'
output: def rock_paper_scissors(player1, player2):
      if player1 == player2:
          return "It's a draw!"
      elif (player1 == 'rock' and player2 == 'scissors') or \
           (player1 == 'paper' and player2 == 'rock') or \
           (player1 == 'scissors' and player2 == 'paper'):
          return 'You win!'
      else:
          return 'You lose!'