input: Write a function called start_game that initializes an interactive text-based RPG game where the player can move around rooms and battle monsters. The function should create a Player object with an initial health of 100 and a starting location of 'room one', and a Room object with a list of monsters. The function should also include a main game loop where the player can choose actions like 'move' to a new room or 'fight' to battle a monster. The game loop should continue until the player's health drops to 0.
  
  assert start_game() is None
output: def start_game():
      class Player:
          def __init__(self, health, location):
              self.health = health
              self.location = location
  
      class Room:
          def __init__(self, monsters):
              self.monsters = monsters
  
      player = Player(100, 'room one')
      room = Room(['monster1', 'monster2'])
  
      while player.health > 0:
          action = input("Choose an action (move/fight): ")
          if action == 'move':
              player.location = 'room two'
              print(f"Moved to {player.location}")
          elif action == 'fight':
              if room.monsters:
                  monster = room.monsters.pop()
                  print(f"Battling {monster}")
                  player.health -= 20
                  print(f"Player health: {player.health}")
              else:
                  print("No monsters left in this room.")
          else:
              print("Invalid action. Try again.")