python: allow to customize Python namespaces
python: allow to customize Python namespaces

- extra methods through PYOPENCV_EXTRA_METHODS_<NS_UPPER> macros
- extra constants through PYOPENCV_EXTRA_CONSTANTS_<NS_UPPER> macros

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv