Merge pull request #20370 from ddacw:stub-gen-next

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

modules/python/common.cmake

@@ -34,6 +34,11 @@ if(TARGET gen_opencv_python_source)
   add_dependencies(${the_module} gen_opencv_python_source)
 endif()
 
+if(TARGET copy_opencv_typing_stubs)
+  # Python 3.6+
+  add_dependencies(${the_module} copy_opencv_typing_stubs)
+endif()
+
 ocv_assert(${PYTHON}_VERSION_MAJOR)
 ocv_assert(${PYTHON}_VERSION_MINOR)