Detailed description

The Python background subtractor tutorial attempts to check that the capture device is opened with:

if not capture.isOpened:
    ...

Instead the method needs to be called:

if not capture.isOpened():
    ...

Currently, it will always be true since the value is a function which is always truthy.

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  answers.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc