Fix numpy include dirs

[Tobias-Fischer]

This PR tackles two issues:

1. Removing deprecated FindPythonInterp and replacing it with FindPython3. We have at least CMake 3.14 on all platforms, and it is Cmake 3.14 that introduced FindPython3.
2. The previous logic only got the (correct) include path on Apple or Windows, but not Linux - on Linux, if the system numpy was found, it was used (which is undesirable in cases like RoboStack where the conda-shipped numpy must be used instead: https://github.com/RoboStack/ros-galactic/issues/23)

/cc @tfoote @wolfv @traversaro @ooeygui

[Tobias-Fischer]

Note that this patch is included in RoboStack and is working fine there.

[sloretz]

The change looks reasonable except that I couldn't figure out if Python3_EXECUTABLE will map to the Debug python executable when appropriate. I kicked off a Windows Debug build to check:

CI (repos: https://gist.githubusercontent.com/sloretz/da9294ef618229c2fe1e6209c297bd78/raw/40dc6e2f23219197263434751c5eb20244b1def6/ros2.repos build type: Debug build: --packages-above-and-dependencies rosidl_generator_py test: --packages-above rosidl_generator_py)

• Windows Debug

[wolfv]

hmm, it says:

01:44:23 -- Using all available rosidl_typesupport_c: rosidl_typesupport_fastrtps_c;rosidl_typesupport_introspection_c
01:44:23 CMake Error at C:/Program Files/CMake/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
01:44:23   Could NOT find Python3 (missing: Python3_NumPy_INCLUDE_DIRS NumPy) (found
01:44:23   version "3.8.3")
01:44:23 Call Stack (most recent call first):
01:44:23   C:/Program Files/CMake/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
01:44:23   C:/Program Files/CMake/share/cmake-3.20/Modules/FindPython/Support.cmake:3165 (find_package_handle_standard_args)
01:44:23   C:/Program Files/CMake/share/cmake-3.20/Modules/FindPython3.cmake:485 (include)
01:44:23   cmake/rosidl_generator_py_generate_interfaces.cmake:20 (find_package)
01:44:23   C:/ci/ws/install/share/ament_cmake_core/cmake/core/ament_execute_extensions.cmake:48 (include)
01:44:23   C:/ci/ws/install/share/rosidl_cmake/cmake/rosidl_generate_interfaces.cmake:292 (ament_execute_extensions)
01:44:23   CMakeLists.txt:56 (rosidl_generate_interfaces)

It might be possible to set teh interpreter directly for teh debug case, with

https://cmake.org/cmake/help/git-stage/module/FindPython3.html#artifacts-specification

set(Python3_EXECUTABLE mypathtopython)

What do you think @sloretz ?

[sloretz]

Yeah, setting the debug interpreter directly seems reasonable. There is logic for finding the Python debug executable in a find module provided by python_cmake_module. It looks like this package already depends on python_cmake_module, so I like the idea of replacing PythonInterp with Python3 in that find module and using the result here.

https://github.com/ros2/rosidl_python/blob/17de859d09fdddc611f003757a9df1acab865cdf/rosidl_generator_py/package.xml#L21

[Tobias-Fischer]

That sounds like a good idea to me @sloretz!

[clalancette]

I just merged in #140, which basically does this and a bunch more. So going ahead and closing this one.