Update FindSphinx.cmake

[AnHeuermann]

Description

FindSphinx.cmake is using removed features:

CMake Warning (dev) at config.cmake/FindSphinx.cmake:145 (find_package):
  Policy CMP0148 is not set: The FindPythonInterp and FindPythonLibs modules
  are removed.  Run "cmake --help-policy CMP0148" for policy details.  Use
  the cmake_policy command to set the policy and suppress this warning.

Call Stack (most recent call first):
  doc/CMakeLists.txt:3 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

Steps to reproduce the behavior

Use a new-ish version of CMake to configure, e.g. by using Docker image ghcr.io/openmodelica/build-deps:v1.26.0:

cmake -S . -B build_cmake

Suggested changes

• Change find_package(PythonInterp QUIET) to find_package(Python3 QUIET COMPONENTS Interpreter)
• Chnage ${PYTHON_EXECUTABLE} to {Python3_EXECUTABLE}

If we find a good solution we could upstream it to https://github.com/cmake-basis/find-modules/blob/master/FindSphinx.cmake where the file originally comes from.

Version and OS

• Version: OMSimulator v3.0.0.post119-gef71d82-linux-debug
• OS: Ubuntu 24.04.3 LTS
• cmake version 3.28.3