How to write a stress test script for asynchronous processes in python
I think there is no need to write your own stress testing script. 
I have used www.blitz.io for stress testing. It is set up in minutes, easy to use and it makes beautiful graphs. 
It has a 14 day trial so you just can test the heck out of your system for 14 days for free. This should be enough to find all your bottlenecks.
I have a web application running on django, wherein an end user can enter a URL to process. All the processing tasks are offloaded to a celery queue which sends notification to the user when the task is completed.
I need to stress test this app with the goals.

to determine breaking points or safe usage limits
to confirm intended specifications are being met
to determine modes of failure (how exactly a system fails)
to test stable operation of a part or system outside standard usage

How do I go about writing my script in Python, given the fact that I also need to take the offloaded celery tasks into account as well.