gh-104212: Explain how to port imp.load_source()

[vstinner]

Explain how to port removed imp "load" functions to Python 3.13 in What's New in Python 3.12.

• Issue: Improve the docs regarding the migration from imp to importlib  #104212

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📚 Documentation preview 📚: https://cpython-previews--105951.org.readthedocs.build/

[vstinner]

I'm not comfortable to add such wall of text just to update code using imp in What's New in Python 3.12. I'm not convinced that init_builtin() or load_dynamic() are commonly used. These functions are more like internal functions used by the imp.load_module() function.

Moreover, I'm not sure how find_module() was used with load_module() to load modules. Is the use case to specify a search path which is not in sys.path? Why not putting the path in sys.path and simply use importlib.import_module()?

But I still consider that imp.load_source() is different and remains a relevant use case on its own. So I proposed PR #105978 to just document a recipe to replace this one.