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

cmake/OpenCVDetectPython.cmake

@@ -297,3 +297,10 @@ elseif(PYTHON3_EXECUTABLE AND PYTHON3INTERP_FOUND)
     set(PYTHON_DEFAULT_AVAILABLE "TRUE")
     set(PYTHON_DEFAULT_EXECUTABLE "${PYTHON3_EXECUTABLE}")
 endif()
+
+if(PYTHON_DEFAULT_AVAILABLE)
+  execute_process(COMMAND ${PYTHON_DEFAULT_EXECUTABLE} --version
+                  OUTPUT_VARIABLE PYTHON_DEFAULT_VERSION
+                  OUTPUT_STRIP_TRAILING_WHITESPACE)
+  string(REGEX MATCH "[0-9]+.[0-9]+.[0-9]+" PYTHON_DEFAULT_VERSION "${PYTHON_DEFAULT_VERSION}")
+endif()