gh-35230: `sage.rings.function_field`: Modularization fixes
    
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### 📚 Description
- We split out `.derivations` (which uses modules) from `.maps` (which
doesn't).
- We separate the implementation of rational function fields (which do
not need Singular for Gröbner basis calculations) from the
implementation of more complicated function fields (which do).
- We move some imports into methods.

This is for modularization purposes:
- Part of #29705



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- Depends on #35237 (for the feature `sage.libs.singular`)
- Depends on #35119 (to preempt merge conflicts)
    
URL: #35230
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

gh-35266: `sage.graphs`: Add `# optional` doctest tags for modularization
    
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These tags make it possible/meaningful to doctest `sage.graphs` even
when some "standard" packages (`networkx`, `numpy`, the modularized
distribution packaging providing `sage.groups`) are not installed.

- Part of #29705
- Split out from #34998

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This is just one commit on top of:
- #35237
    
URL: #35266
Reported by: Matthias Köppe
Reviewer(s): David Coudert, Dima Pasechnik, Matthias Köppe

gh-35267: `sage.manifolds`, `sage.tensor`: Add `# optional` doctest tags for modularization
    
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Adding tags that make it possible/meaningful to doctest
`sage.manifolds`, `sage.tensor` even when some "standard" packages
(`pplpy`, the modularized distribution packaging providing
`sage.symbolic` and `sage.manifolds`) are not installed. Also taking
care of `sage.geometry.riemannian_manifolds`.

- Part of #29705
- Split out from #34998

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This is just two commits on top of:
- #35237
    
URL: #35267
Reported by: Matthias Köppe
Reviewer(s): Eric Gourgoulhon

sagemathgh-35396: Activate/fix rpy2 tests
    
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As a follow-up on:
- sagemath#35237

we make `rpy2` a dynamically detected feature, enabling the automatic
run of doctests tagged `# optional - rpy2`; we use a file-level doctest
tag for the `r.py` file.

We also update the spkg documentation for sagemath#35347

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Resolves sagemath#35347

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URL: sagemath#35396
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

sagemathgh-35396: Activate/fix rpy2 tests
    
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multiply two integers" -->

### 📚 Description

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As a follow-up on:
- sagemath#35237

we make `rpy2` a dynamically detected feature, enabling the automatic
run of doctests tagged `# optional - rpy2`; we use a file-level doctest
tag for the `r.py` file.

We also update the spkg documentation for sagemath#35347

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example "Fixes sagemath#12345". -->

Resolves sagemath#35347

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### ⌛ Dependencies

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URL: sagemath#35396
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

sagemathgh-35396: Activate/fix rpy2 tests
    
<!-- Please provide a concise, informative and self-explanatory title.
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below. -->
<!-- For example, instead of "Fixes sagemath#12345", use "Add a new method to
multiply two integers" -->

### 📚 Description

<!-- Describe your changes here in detail. -->

As a follow-up on:
- sagemath#35237

we make `rpy2` a dynamically detected feature, enabling the automatic
run of doctests tagged `# optional - rpy2`; we use a file-level doctest
tag for the `r.py` file.

We also update the spkg documentation for sagemath#35347

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example "Fixes sagemath#12345". -->

Resolves sagemath#35347

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### ⌛ Dependencies

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- Depends on sagemath#35237
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URL: sagemath#35396
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

sagemathgh-35396: Activate/fix rpy2 tests
    
<!-- Please provide a concise, informative and self-explanatory title.
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<!-- Don't put issue numbers in the title. Put it in the Description
below. -->
<!-- For example, instead of "Fixes sagemath#12345", use "Add a new method to
multiply two integers" -->

### 📚 Description

<!-- Describe your changes here in detail. -->

As a follow-up on:
- sagemath#35237

we make `rpy2` a dynamically detected feature, enabling the automatic
run of doctests tagged `# optional - rpy2`; we use a file-level doctest
tag for the `r.py` file.

We also update the spkg documentation for sagemath#35347

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example "Fixes sagemath#12345". -->

Resolves sagemath#35347

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### ⌛ Dependencies

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- Depends on sagemath#35237
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URL: sagemath#35396
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe