datamodel-code-generator: Fix NixOS entry

[wentasah]

This fixes the entry added recently in #51568 by @arjo129.

NixOS and Nixpkgs provide Python packages through multiple package sets:

• Version-specific sets, such as python312Packages for Python 3.12
• The generic set python3Packages, which always points to the default Python version in Nixpkgs.

For rosdep, it is preferable to use the generic set, since version-specific sets are eventually removed from Nixpkgs.

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For changes related to rosdep:

• ✅ Platforms for new rosdep rules are valid

For changes related to yamllint:

• ✅ All new lines of YAML pass linter checks

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