CMake: use FindPython as opposed to FindPythonLibs

[KrisThielemans]

Python support has changed dramatically in CMake, so we need to update our files

STIR/src/swig/CMakeLists.txt

Lines 94 to 99 in aae4c92

	find_package(PythonInterp)
	# find libraries and include files
	# TODO would be better to use target_include_directories
	FIND_PACKAGE(PythonLibs REQUIRED)
	INCLUDE_DIRECTORIES(${PYTHON_INCLUDE_PATH})
	FIND_PACKAGE(Numpy REQUIRED)

It would for instance enable pypy support since CMake 3.18.1 (https://cmake.org/cmake/help/v3.18/module/FindPython.html)

Unfortunately it means handling differences between PYTHON_EXECUTABLE and Python_EXECUTABLE, see also SyneRBI/SIRF-SuperBuild#313

[KrisThielemans]

This was done in #985