gh-35237: sage.features: Add sage.libs.singular, features for sta…

…ndard Python packages

    
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### 📚 Description
This is for the modularization effort, in particular:
- #35095

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We also add missing copyright headers and update existing copyrights in
`sage.features`.

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### ⌛ Dependencies
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URL: #35237
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee