Python 3.9 will be released approximately 2020-10-05. If we want to maintain a standard of only supporting the last 3 Python minor versions, we should begin testing that Streamlit works with Python 3.9.

Python 3.9 RC1
https://www.python.org/downloads/release/python-390rc1/