sagemathgh-37654: Developer guide: Improve discussion of backport packages
    
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- sagemath#35203 added these packages as unconditional dependencies
- prompted by a sporadic use of typing_extensions in the Sage library
(see sagemath#34831)
- motivated further by the demand to immediately drop support for Python
3.8 so that newer typing features can be used in the Sage library
(sagemath#35404)
- sagemath#36776 reduced the packages to
conditional dependencies

Here we improve the documentation in the section "Language standard" of
the developer's guide so that it aligns with how the conditional
dependencies are declared.

Based on changes split out from sagemath#37399.

### 📝 Checklist

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### ⌛ Dependencies

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URL: sagemath#37654
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe