System information (version)

• OpenCV => 4.1.0
• Operating System / Platform => macOS 10.15
• Compiler => homebrew

Detailed description

I get a segmentation fault when calling a python class which inherits from a openCV-cpp-class (similar to the issue described here: https://github.com/microsoft/ptvsd/issues/1410). This makes extending openCV-classes in Python unreliable. I'm not even sure if it should be possible to inherit openCV classes (as superclass) in Python because the .create()-policy to instantiate a new instance will always return the pure cpp-class not the inherited python class. Instantiation without .create() (like foo = PyDerivedClass()) will lead to a TypeError (incorrect type of self ...) when trying to call the inherited cpp-methods.

All in all, it might be worth rethinking which classes etc. should have python bindings (e.g. cv2.ml_StatModel can be called in python but its pretty much useless) and/or make a note somewhere in the documentation.

Steps to reproduce

```.py
import cv2
class DerivedClass(cv2.ml_NormalBayesClassifier):
    def __init__(self):
        super().__init__()

for i in range(3):
    foo = DerivedClass()

```