Replace CMake Python variables with new ones from FindPython3 module

CMakeLists.txt

@@ -95,16 +95,24 @@ endif()
 if (SKIP_PYBIND11)
   message(STATUS "SKIP_PYBIND11 set - disabling python bindings")
 else()
-  include(IgnPython)
-  find_package(PythonLibs QUIET)
-  if (NOT PYTHONLIBS_FOUND)
-    IGN_BUILD_WARNING("Python is missing: Python interfaces are disabled.")
-    message (STATUS "Searching for Python - not found.")
+  #include(IgnPython) TODO: allow to specify for what it should search and then
+  # the code below can be removed; e.g. pybind needs Interpreter and Development components
+  # see https://pybind11.readthedocs.io/en/stable/cmake/index.html#new-findpython-mode
+  if(${CMAKE_VERSION} VERSION_LESS "3.12.0")
+    find_package(PythonLibs QUIET)
+    set(Python3_FOUND ${PYTHONLIBS_FOUND})
+    set(Python3_VERSION ${PYTHONLIBS_VERSION_STRING})
   else()
-    message (STATUS "Searching for Python - found version ${PYTHONLIBS_VERSION_STRING}.")
+    find_package(Python3 QUIET COMPONENTS Interpreter Development)
+  endif()
+
+  if (NOT Python3_FOUND)
+    IGN_BUILD_WARNING("Python3 is missing: Python interfaces are disabled.")
+    message (STATUS "Searching for Python3 - not found.")
+  else()
+    message (STATUS "Searching for Python3 - found version ${Python3_VERSION}.")
 
     set(PYBIND11_PYTHON_VERSION 3)
-    find_package(Python3 QUIET COMPONENTS Interpreter Development)
     find_package(pybind11 2.2 QUIET)
 
     if (${pybind11_FOUND})

src/python_pybind11/CMakeLists.txt

@@ -1,8 +1,11 @@
-if(WIN32 AND CMAKE_BUILD_TYPE STREQUAL "Debug")
-  # pybind11 logic for setting up a debug build when both a debug and release
-  # python interpreter are present in the system seems to be pretty much broken.
-  # This works around the issue.
-  set(PYTHON_LIBRARIES "${PYTHON_DEBUG_LIBRARIES}")
+if(${CMAKE_VERSION} VERSION_LESS "3.12.0")
+  # TODO: remove once the minimum CMake version is increased
+  if(WIN32 AND CMAKE_BUILD_TYPE STREQUAL "Debug")
+    # pybind11 logic for setting up a debug build when both a debug and release
+    # python interpreter are present in the system seems to be pretty much broken.
+    # This works around the issue.
+    set(PYTHON_LIBRARIES "${PYTHON_DEBUG_LIBRARIES}")
+  endif()
 endif()
 
 message(STATUS "Building pybind11 interfaces")
@@ -56,17 +59,15 @@ target_link_libraries(math PRIVATE
 )
 
 if(USE_SYSTEM_PATHS_FOR_PYTHON_INSTALLATION)
+  # Get install variable from Python3 module
   if(${CMAKE_VERSION} VERSION_LESS "3.12.0")
+    # Python3_SITEARCH is available via FindPython3 from 3.12 on, workaround if needed
     execute_process(
-      COMMAND "${PYTHON_EXECUTABLE}" -c "if True:
+      COMMAND "${Python3_EXECUTABLE}" -c "if True:
   from distutils import sysconfig as sc
   print(sc.get_python_lib(plat_specific=True))"
       OUTPUT_VARIABLE Python3_SITEARCH
       OUTPUT_STRIP_TRAILING_WHITESPACE)
-  else()
-    # Get install variable from Python3 module
-    # Python3_SITEARCH is available from 3.12 on, workaround if needed:
-    find_package(Python3 COMPONENTS Interpreter)
   endif()
 
   if(USE_DIST_PACKAGES_FOR_PYTHON)