gh-140196: Added constructor behavior changes in ast.rst for python 3.13

[MannXo]

Summary

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This adds a .. versionchanged:: 3.13 note to ast.rst documenting that AST node constructors now provide sensible defaults for omitted fields in Python 3.13: optional fields default to None, list fields default to an empty list, and fields of type :class:ast.expr_context default to :class:Load() <ast.Load>

Issue

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#140196

• Issue: Document missing AST node constructor behavior changes in ast.rst for python 3.13 #140196

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📚 Documentation preview 📚: https://cpython-previews--140243.org.readthedocs.build/

[python-cla-bot]

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