input: Write a python function to predict future stock prices using a rolling window approach.
  assert predict_stock_prices([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15], 3)==[2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0]
  assert predict_stock_prices([5, 10, 15, 20, 25, 30], 2)==[7.5, 12.5, 17.5, 22.5, 27.5]
  assert predict_stock_prices([100, 105, 110, 115, 120, 125], 4)==[107.5, 110.0, 112.5]
output: def predict_stock_prices(prices, window_size):
      predictions = []
      for i in range(len(prices) - window_size):
          window = prices[i:i+window_size]
          avg_price = sum(window) / window_size
          predictions.append(avg_price)
      return predictions