Use the modern FindPython instead of FindPython3 (#2881)

cmake/ExecuteFindPython/CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.15 FATAL_ERROR)
 # This is called from PythonHelper.cmake in a subprocess, to allow multiple Python versions to be
 # searched for in the project without CACHE variable hackery.
 project(ExecuteFindPython LANGUAGES C)
-find_package(Python3 COMPONENTS ${Python3_COMPONENTS})
-message(STATUS "Python3_INCLUDE_DIRS=${Python3_INCLUDE_DIRS}")
-message(STATUS "Python3_LIBRARIES=${Python3_LIBRARIES}")
-message(STATUS "Python3_VERSION=${Python3_VERSION_MAJOR}.${Python3_VERSION_MINOR}")
+find_package(Python COMPONENTS ${Python_COMPONENTS})
+message(STATUS "Python_INCLUDE_DIRS=${Python_INCLUDE_DIRS}")
+message(STATUS "Python_LIBRARIES=${Python_LIBRARIES}")
+message(STATUS "Python_VERSION=${Python_VERSION_MAJOR}.${Python_VERSION_MINOR}")

cmake/PythonHelper.cmake

@@ -72,7 +72,7 @@ function(nrn_find_python)
     message(FATAL_ERROR "${opt_KEYWORDS_MISSING_VALUES} values are required")
   endif()
   if(NOT IS_ABSOLUTE "${opt_NAME}")
-    # Find the full path to ${opt_NAME} as Python3_EXECUTABLE does not accept relative paths.
+    # Find the full path to ${opt_NAME} as Python_EXECUTABLE does not accept relative paths.
     find_program(
       "${opt_NAME}_full" "${opt_NAME}"
       PATHS ENV PATH
@@ -87,14 +87,14 @@ function(nrn_find_python)
   endif()
   # Only bother finding version/include/library information if NRN_ENABLE_PYTHON is set.
   if(NRN_ENABLE_PYTHON)
-    # Run find_package(Python3 ...) in a subprocess, so there is no pollution of CMakeCache.txt and
+    # Run find_package(Python ...) in a subprocess, so there is no pollution of CMakeCache.txt and
     # so on. Our desire to include multiple Python versions in one build means we have to handle
     # lists of versions/libraries/... manually. Unfortunately one cannot safely use find_package in
     # CMake script mode, so we configure an extra project.
     string(SHA1 pyexe_hash "${opt_NAME}")
     string(SUBSTRING "${pyexe_hash}" 0 6 pyexe_hash)
     # Which attributes we're trying to learn about this Python
-    set(python_vars Python3_INCLUDE_DIRS Python3_VERSION)
+    set(python_vars Python_INCLUDE_DIRS Python_VERSION)
     if(NRN_ENABLE_PYTHON_DYNAMIC AND NOT NRN_LINK_AGAINST_PYTHON)
       # Do not link against Python, so we don't need the library -- just as well, it's not available
       # in manylinux
@@ -105,15 +105,15 @@ function(nrn_find_python)
         )
       endif()
       set(dev_component "Development.Module")
-      set(Python3_LIBRARIES "do-not-link-against-libpython-in-dynamic-python-builds")
+      set(Python_LIBRARIES "do-not-link-against-libpython-in-dynamic-python-builds")
     else()
       set(dev_component "Development")
-      list(APPEND python_vars Python3_LIBRARIES)
+      list(APPEND python_vars Python_LIBRARIES)
     endif()
     execute_process(
       COMMAND
-        ${CMAKE_COMMAND} "-DPython3_EXECUTABLE:STRING=${opt_NAME}"
-        "-DPython3_COMPONENTS=${dev_component};Interpreter" -S
+        ${CMAKE_COMMAND} "-DPython_EXECUTABLE:STRING=${opt_NAME}"
+        "-DPython_COMPONENTS=${dev_component};Interpreter" -S
         ${CMAKE_SOURCE_DIR}/cmake/ExecuteFindPython -B
         ${CMAKE_BINARY_DIR}/ExecuteFindPython_${pyexe_hash}
       RESULT_VARIABLE result
@@ -132,13 +132,13 @@ function(nrn_find_python)
       set(${var} "${CMAKE_MATCH_1}")
     endforeach()
     set("${opt_PREFIX}_INCLUDES"
-        "${Python3_INCLUDE_DIRS}"
+        "${Python_INCLUDE_DIRS}"
         PARENT_SCOPE)
     set("${opt_PREFIX}_LIBRARIES"
-        "${Python3_LIBRARIES}"
+        "${Python_LIBRARIES}"
         PARENT_SCOPE)
     set("${opt_PREFIX}_VERSION"
-        "${Python3_VERSION}"
+        "${Python_VERSION}"
         PARENT_SCOPE)
   endif()
   # Finally do our special treatment for macOS + sanitizers
@@ -202,7 +202,7 @@ foreach(pyexe ${python_executables})
       message(FATAL_ERROR "Cannot handle multiple Python include dirs: ${nrnpy_INCLUDES}")
     endif()
     if(NOT num_lib_dirs EQUAL 1)
-      message(FATAL_ERROR "Cannot handle multiple Python libraries: ${Python3_LIBRARIES}")
+      message(FATAL_ERROR "Cannot handle multiple Python libraries: ${Python_LIBRARIES}")
     endif()
     if(nrnpy_VERSION IN_LIST NRN_PYTHON_VERSIONS)
       # We cannot build against multiple copies of the same pythonX.Y version.