sagemathgh-37486: sagemath-{objects,categories} `pyproject.toml`: Add 'external' section according to draft PEP 725
    
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As done in sagemath#37482 (unbundled from sagemath#37446) for **sagemath-standard**.

- This information will be used in the skeleton generator in
pyodide/pyodide#4438

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### ⌛ Dependencies

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URL: sagemath#37486
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee