input: Write a python function to determine the winner of a rock, paper, scissors game.
  assert determine_winner('rock', 'scissors') == 'You win!'
  assert determine_winner('rock', 'paper') == 'Computer wins!'
  assert determine_winner('paper', 'rock') == 'You win!'
  assert determine_winner('paper', 'scissors') == 'Computer wins!'
  assert determine_winner('scissors', 'rock') == 'Computer wins!'
  assert determine_winner('scissors', 'paper') == 'You win!'
  assert determine_winner('rock', 'rock') == 'It\'s a tie!'
  assert determine_winner('paper', 'paper') == 'It\'s a tie!'
  assert determine_winner('scissors', 'scissors') == 'It\'s a tie!'
output: def determine_winner(player, computer):
      if player == computer:
          return "It's a tie!"
      
      if (player == 'rock' and computer == 'scissors') or \
         (player == 'paper' and computer == 'rock') or \
         (player == 'scissors' and computer == 'paper'):
          return 'You win!'
      else:
          return 'Computer wins!'