feat: move Pythonizations implementations to source files

[wdconinc]

This PR moves the implementation of Pythonizations detail functions into a source file. This allows the headers to be included in C++20 modules (#907).

BEGINRELEASENOTES

• Move Pythonizations detail implementations to separate source file

ENDRELEASENOTES

[Copilot]

Pull request overview

This PR refactors the Pythonizations implementation by moving function definitions from inline functions in the header file to a separate source file. This change enables the headers to be included in C++20 modules while maintaining the same functionality.

• Function implementations moved from static inline in header to regular definitions in source file
• Added target_link_libraries(podio PRIVATE Python3::Python) to CMakeLists.txt
• Header file now contains only forward declarations

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/Pythonizations.cc	New source file containing implementations of subscript and pythonize_subscript functions
include/podio/detail/Pythonizations.h	Refactored to contain only function declarations, removed inline implementations and <stdexcept> include
src/CMakeLists.txt	Added Pythonizations.cc to core_sources and linked Python3 library

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[wdconinc]

Ah, this probably should have been in the Python:: namespace since that's what is added as a dependency in the config

podio/cmake/podioConfig.cmake.in

Lines 28 to 32 in 5d82748

	if(NOT "@REQUIRE_PYTHON_VERSION@" STREQUAL "")
	find_dependency(Python @REQUIRE_PYTHON_VERSION@ COMPONENTS Interpreter Development)
	else()
	find_dependency(Python COMPONENTS Interpreter Development)
	endif()

and in the PODIO_GENERATE_DATAMODEL module:

podio/cmake/podioMacros.cmake

Line 158 in 5d82748

COMMAND ${Python_EXECUTABLE} ${podio_PYTHON_DIR}/podio_class_generator.py ${CLANG_FORMAT_ARG} ${SCHEMA_EVOLUTION_ARG} ${UPSTREAM_EDM_ARG} ${YAML_FILE} ${ARG_OUTPUT_FOLDER} ${datamodel} ${ARG_IO_BACKEND_HANDLERS} ${LANGUAGE_ARG} ${VERSION_ARG} ${OLD_DESCRIPTION_ARG}

[wdconinc]

Then again, the package that is found is Python3...

podio/cmake/podioBuild.cmake

Lines 197 to 200 in 5d82748

	if(NOT PODIO_RELAX_PYVER)
	find_package(Python3 ${REQUIRE_PYTHON_VERSION} EXACT REQUIRED COMPONENTS Development Interpreter)
	else()
	find_package(Python3 ${REQUIRE_PYTHON_VERSION} REQUIRED COMPONENTS Development Interpreter)