input: Write a python function to play a game of Rock-Paper-Scissors.
  assert play_rps('rock', 'scissors') == "User won!"
  assert play_rps('rock', 'paper') == "Computer won!"
  assert play_rps('rock', 'rock') == "It's a tie!"
  assert play_rps('paper', 'scissors') == "Computer won!"
  assert play_rps('paper', 'rock') == "User won!"
  assert play_rps('scissors', 'scissors') == "It's a tie!"
output: def play_rps(user_choice, computer_choice):
      if user_choice == computer_choice:
          return "It's a tie!"
      elif (user_choice == 'rock' and computer_choice == 'scissors') or \
           (user_choice == 'paper' and computer_choice == 'rock') or \
           (user_choice == 'scissors' and computer_choice == 'paper'):
          return "User won!"
      else:
          return "Computer won!"