As of today, new Deployments using Python Serverless Functions will use version 3.9 and the legacy version 3.6 is being deprecated.
If you need to continue making Deployments using Python 3.6, ensure your Pipfile and corresponding Pipfile.lock have python_version set to 3.6 exactly.
Python 3.6 will reach end of life in December 2021. Before completely removing support, there will be another announcement with the exact sunset date.
Check out the documentation as well.