As noted in other topics, this is supported in the Apache HTTP client jars that come with Ignition. Like this one:
Your code is missing something or doing something wrong and it isn't clear based on what you've shared.
Your IT department is simply wrong. Use a packet capture to compare to working requests so you can see the differences. I would suspect some aspect of how your gateway service account is set up, but the possibilities are endless. (Are you running your Python3 test code in a service with the same account as Ignition? Desktop and service environments are different.)