doc: change default python version for sphinx to 3
Change FindSphinx to find  python 3
## POSSIBILITY OF SUCH DAMAGE.
set(_python_paths)
-find_package(PythonInterp)
+find_package(PythonInterp 3)
if(PYTHON_EXECUTABLE)
get_filename_component(_python_dir "${PYTHON_EXECUTABLE}" DIRECTORY)
list(APPEND _python_paths
