It is possible to opt out of dark mode using a key in the Info.plist file. This can also be used in the Python.app hidden in the framework to globally disable support for dark mode.

I wouldn't switch back to 8.6.9, if only because that doesn't build properly on macOS 11 (and hence would make building the "univeral2" installer a lot harder). 

I'd rather use the tip of the 8.6.x branch for Tk to get the latest bug fixes (probably picking a specific commit to be able to reproduce the build).