Python typing stub generation #20370
Add stub generation to `gen2.py`, addressing #14590.
### Pull Request Readiness Checklist
See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request
- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
- [x] The PR is proposed to proper branch
- [x] There is reference to 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
[G-API] Introduce GOpaque and GArray for python
* Introduce GOpaque and GArray for python
* Fix ctor
* Avoid code duplication by using macros
* gapi: move Python-specific files to misc/python
* Fix windows build
Co-authored-by: Alexander Alekhin <alexander.a.alekhin@gmail.com>