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

sagemathgh-37486: sagemath-{objects,categories} `pyproject.toml`: Add 'external' section according to draft PEP 725
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

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

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#37486
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee