I've downloaded versions of OpenCV 4.1.2 a few days apart. The earlier version has a cmake-gui option:
BUILD_opencv_python3:BOOL=ON
It is not present in today's version (2019 nov 04). It's not that it's switched off--the option does not appear at all. I'm not sure of its exact function (anyone?) but since it had defaulted to ON in the previous download, it sounded significant.
Another new flag is present in today's build:
OPENCV_ENABLE_MEMALIGN:BOOL=ON
I'm not as concerned about that, but it does indicate that some things have changed.