input: Write a python function to determine the winner of a rock-paper-scissors game without using random choices.
  assert play_game('rock', 'scissors') == 'Player 1 wins!'
  assert play_game('rock', 'paper') == 'Player 2 wins!'
  assert play_game('rock', 'rock') == 'It\'s a tie!'
  assert play_game('paper', 'rock') == 'Player 1 wins!'
  assert play_game('paper', 'scissors') == 'Player 2 wins!'
  assert play_game('paper', 'paper') == 'It\'s a tie!'
  assert play_game('scissors', 'rock') == 'Player 2 wins!'
  assert play_game('scissors', 'paper') == 'Player 1 wins!'
  assert play_game('scissors', 'scissors') == 'It\'s a tie!'
output: def play_game(player1, player2):
      if player1 == player2:
          return "It's a tie!"
      elif (player1 == 'rock' and player2 == 'scissors') or \
           (player1 == 'paper' and player2 == 'rock') or \
           (player1 == 'scissors' and player2 == 'paper'):
          return 'Player 1 wins!'
      else:
          return 'Player 2 wins!'