python: accept path-like objects wherever file names are expected

[tailsu]

Merry Christmas, all 🎄

Implements #15731

Support is enabled for all arguments named filename or filepath (case-insensitive), or annotated with CV_WRAP_FILE_PATH.

Support is based on PyOS_FSPath, which is available in Python 3.6+. When running on older Python versions the arguments must have a str value as before.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• 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]

OpenCV team build OpenCV-Python package with Limited API option to reduce support matrix. The patch breaks it:

[ 97%] Building CXX object modules/gapi/CMakeFiles/opencv_test_gapi.dir/test/internal/gapi_int_perform_substitution_test.cpp.o
/mnt/projects/Projects/OpenCV/opencv-master/modules/python/src2/cv2_convert.cpp: In function ‘bool pyopencv_to(PyObject*, T&, const ArgInfo&) [with T = std::__cxx11::basic_string<char>; PyObject = _object]’:
/mnt/projects/Projects/OpenCV/opencv-master/modules/python/src2/cv2_convert.cpp:708:15: error: ‘PyOS_FSPath’ was not declared in this scope
         obj = PyOS_FSPath(obj);
               ^~~~~~~~~~~
/mnt/projects/Projects/OpenCV/opencv-master/modules/python/src2/cv2_convert.cpp:708:15: note: suggested alternative: ‘Py_SetPath’
         obj = PyOS_FSPath(obj);
               ^~~~~~~~~~~
               Py_SetPath
modules/python3/CMakeFiles/opencv_python3.dir/build.make:117: recipe for target 'modules/python3/CMakeFiles/opencv_python3.dir/__/src2/cv2_convert.cpp.o' failed
make[2]: *** [modules/python3/CMakeFiles/opencv_python3.dir/__/src2/cv2_convert.cpp.o] Error 1

[asmorkalov]

Fix for the versioning issue of limited API: #24837

[asmorkalov]

👍

[opencv-alalek]

Need to update this check, because compilation fails in this configuration:

--   Python 3:
--     Interpreter:                 C:/utils/soft/python35-x64/python.exe (ver 3.5.4)
--     Libraries:                   C:/utils/soft/python35-x64/Libs/python35.lib (ver 3.5.4)
--     Limited API:                 YES (ver 0x03060000)
--     numpy:                       C:/utils/soft/python35-x64/lib/site-packages/numpy/core/include (ver 1.12.1)
--     install path:                python/cv2/python-3

https://pullrequest.opencv.org/buildbot/builders/precommit_opencl/builds/100307