CodeRunner runs whatever version of python3 is installed on the Jobe server. So I take you to be saying that when you write a CodeRunner question, the version of python that gets used is python 3.5. Which suggests that your jobe server is due for an upgrade. It's probably running on an older version of Linux.
While it's possible to upgrade the version of Python on an older version of Linux it's much easier just to create a completely new Jobe server starting with Ubuntu 20:04. That should give you python 3.8.5.