Try moving up to a more recent version of Python. It could be that 3.6 is too old for Big Sur, but a newer version will be just fine. It should also keep working with the book but let me know if it fails on some of the exercises (but be triple sure it’s a python version problem and not your fault first).
That’s really garbage. I mean, I’m glad you got it working, but that tells me Apple is ganking people’s python installs to force them to upgrade. Microsoft does something similar with making the installer fail to install python but telling you to install it from the app store to boost app store uses.
Anyway, good to get it working, but sad that was the solution.