The http status codes are returned by the server, not the Python modules. So one probably cause is that urllib and requests are doing something slightly differently, which the server doesn't know how to handle.

My recommendation would be to use something like Fiddler to check the communication using both requests and urllib, to see if if there are any differences in e.g. the headers.

I had some help and we finally figured it out, I needed to have some value in the reader parameter and also I had to open up some permissions for the webhook token through token manager even though I assigned the permissions to the token when I created the webhook. I think there may be something buggy about my install of FME Server but it is working now, thanks for the response.