input: Write a python function to determine the winner in a rock-paper-scissors game.
  assert calculate_winner('rock', 'scissors') == 'player'
  assert calculate_winner('paper', 'rock') == 'player'
  assert calculate_winner('scissors', 'paper') == 'player'
  assert calculate_winner('rock', 'rock') == 'draw'
  assert calculate_winner('scissors', 'rock') == 'computer'
output: def calculate_winner(player_choice, computer_choice):
      if player_choice == computer_choice:
          return 'draw'
      elif (player_choice == 'rock' and computer_choice == 'scissors') or \
           (player_choice == 'paper' and computer_choice == 'rock') or \
           (player_choice == 'scissors' and computer_choice == 'paper'):
          return 'player'
      else:
          return 'computer'