Best way to 'listen' to s3 bucket for new file?
David here, from the Zapier Platform team. 
Seems like you've already found your answer, which is great. I just wanted to plug Zapier as an option (since you had mentioned you're already using it). Our S3 integration has a "new file in bucket" trigger, which you can combine with any other step (such as a Python Code step). Additionally, you could skip the middleman and structure your zap as:

some trigger
Add file to S3
Run Python

And not need to worry about webhooks at all.
​Let me know if you've got any other questions!
I have connected zapier to a webhook I am listening too, which sends a JSON file into my s3 bucket. 
I have some python code that I want to execute when a new file is uploaded into the bucket, in real time over the file. 
What is the best way to 'listen' for the upload of this file into the s3 bucket?