Changing Torah's Identity in Scrapy over Polipo

I am trying to create a CrawlSpider using Scrapy for crawling sites in Tor.

For this, I use Polipo proxy as described in this guide:

http://blog.privatenode.in/torifying-scrapy-project-on-ubuntu/

So now my spider is running and I can access .onion sites in Scrapy. However, I would like to send a NEWNYM signal for each request to change the Tor ID.

How can this be done in Scrapy?