Updated modules in find_package command for Python3

[NicoleJurjew]

No description provided.

[KrisThielemans]

https://cmake.org/cmake/help/latest/module/FindPythonInterp.html deprecated since 3.12, while we require 3.16.2.

Current change is minimal. We probably should pass Python_EXECUTABLE as opposed to PYTHON_EXECUTABLE to dependencies, but this needs a check which ones we use. Probably easiest to pass both. I think we should no longer attempt to pass include and other paths, and let CMake sort it out.

[KrisThielemans]

Need to update how we look for cython I guess

[100%] Generating build/timestamp
Traceback (most recent call last):
  File "/home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/builds/TomoPhantom/build/Wrappers/Python/setup.py", line 19, in <module>
    from Cython.Distutils import build_ext
ModuleNotFoundError: No module named 'Cython'
gmake[5]: *** [Wrappers/Python/CMakeFiles/PythonWrapper.dir/build.make:76: Wrappers/Python/build/timestamp] Error 1
gmake[4]: *** [CMakeFiles/Makefile2:157: Wrappers/Python/CMakeFiles/PythonWrapper.dir/all] Error 2
gmake[3]: *** [Makefile:136: all] Error 2
gmake[2]: *** [CMakeFiles/TomoPhantom.dir/build.make:86: builds/TomoPhantom/stamp/TomoPhantom-build] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:755: CMakeFiles/TomoPhantom.dir/all] Error 2

This error is different from why I saw in #932 but it's possible that @NicoleJurjew only saw the Tomophantom error when using this branch I guess.

[NicoleJurjew]

With the suggested changes, the PYTHONPATH isn't exported correctly.

I think some changes need to be done here, as well:
https://github.com/SyneRBI/SIRF-SuperBuild/blob/5f1a7f498bf6f41551c19eb53798b5d33af43eec/SuperBuild.cmake#L366C1-L386C8

[KrisThielemans]

that's a bit strange. What is the content of env_sirf.sh etc then? Can you just do a

message(STATUS "PYTHON_EXECUTABLE=${PYTHON_EXECUTABLE}")

before those statement to check what it is?

[NicoleJurjew]

This is the content of env_sirf.sh:

Where will that message be written to then? Thanks!