Deadline exceeded while waiting for HTTP response from URL is actually a DeadlineExceededError. When you are making a HTTP request, App Engine maps this request to URLFetch. URLFetch has its own deadline that is configurable. See the URLFetch documentation.
It seems that your client library catches DeadlineExceededError and throws HTTPException. Your client library either passes a deadline to URLFetch (which you would need to change) or the default deadline is insufficient for your request.
Try setting the default URLFetch deadline like this:
from google.appengine.api import urlfetch
urlfetch.set_default_fetch_deadline(45)
Also make sure to check out Dealing with DeadlineExceededErrors in the official Docs.
Keep in mind that any end-user initiated request must complete in 60 seconds or will encounter a DeadlineExceededError. See App Engine Python Runtime Request Timer.