fix: preserve NumPY writeable flag in output arguments

[VadimLevin]

Example error output:

OpenCV(4.8.0-dev) :-1: error: (-5:Bad argument) in function 'rectangle'
> Overload resolution failed:
>  - img marked as output argument, but provided NumPy array marked as readonly
>  - Expected Ptr<cv::UMat> for argument 'img'
>  - img marked as output argument, but provided NumPy array marked as readonly
>  - Expected Ptr<cv::UMat> for argument 'img'

resolves opencv/opencv-python#859

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• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[asmorkalov]

👍