@seanm I don’t have a Mac but I am in the process of updating Python to Python 3.9 on Windows and Kubuntu.
If I update Python but don’t rebuild VTK to pick up the new version of Python I get this error when running a python script:
ModuleNotFoundError: No module named '_vtkmodules_static'
Once I rebuild VTK it is all Ok. When rebuilding VTK for a new version of Python I usually remove all Python related files then run CMake again.
Hope this helps.