Thanks @Kiran_Hegde, this is indeed a bug. I filed a fix for it here: https://jira.mongodb.org/browse/PYTHON-2378
I also have a proposed fix that can be released shortly, here: PYTHON-2378 Use PUT for EC2 token request, not POST by ShaneHarvey · Pull Request #2 · mongodb/pymongo-auth-aws · GitHub
It would be great if you could test out this fix. To test it yourself, install the updated version like this:
python -m pip install --upgrade https://github.com/ShaneHarvey/pymongo-auth-aws/archive/PYTHON-2378.tar.gz