Yep, I found, it is statuses/show
endpoint you're trying to access.
You don't need to hardcode a link and authentication, it is all already done in tweepy. Simply make sure to use its latest version (with 1.1 bindings), and you'll get access to this endpoint with get_status
method:
api = tweepy.api(auth)
tweet = api.get_status(id=tweet_id)
Note that this tweet will be a tweepy tweet model object, and you can access it's fields as
ir2id = tweet.in_reply_to_status_id