(Copying the comment suggestion that worked as the answer.)
At least update from 3.6.3, released 2017-10-3, to at least 3.6.8, released 2018-12-24, or one of the later 3.6 releases. Marzie said that this worked.
I don't know why this worked, but Apple is willing to break 3rd party apps, especially if not written in their custom system language. Tcl/tk, the base GUI framework for Python's tkinter, need relatively frequent changes to keep up with Apple's graphics and UI changes. So upgrading Python (and tcl/tk and tkinter and IDLE) are more important on macOS that on Linux and Windows. I believe maxOS 11 on Apple's CPUs requires 3.10.
Loading more than one version of Python works fine for me. Each version gets its own icons and terminal command (python3.6 for 3.6).