sagemathgh-35396: Activate/fix rpy2 tests

    
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### 📚 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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Resolves sagemath#35347

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- [x] The title is concise, informative, and self-explanatory.
- [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

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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

build/pkgs/r/SPKG.rst

@@ -24,3 +24,14 @@ Upstream Contact
 
 -  https://www.r-project.org
 -  R mailing list, #R in IRC
+
+
+Special Installation Instructions
+---------------------------------
+
+In the Sage distribution, ``r`` is a "dummy" package:
+It is here to provide information about equivalent system packages.
+R cannot be installed using the Sage distribution.
+Please install it manually, either using one of the system package
+commands shown here or following the upstream instructions
+at https://www.r-project.org

build/pkgs/r/distros/conda.txt

@@ -1,2 +1,3 @@
 r
 r-essentials
+r-lattice

build/pkgs/rpy2/SPKG.rst

@@ -12,12 +12,16 @@ License
 -------
 
 -  GPL 2+
--  Note that we have deleted references to Mozilla PL as an option,
-   which we are allowed to do by the full rpy2 license in order to
-   remain GPL-compatible
-
 
 Upstream Contact
 ----------------
 
--  https://rpy2.bitbucket.io
+- https://github.com/rpy2/rpy2
+
+Special Installation Instructions
+---------------------------------
+
+In the Sage distribution, ``rpy2`` is a "semi-standard" package: It will be
+automatically installed by the Sage distribution if a suitable system
+installation of R is detected by ``configure``. (Note that Sage no longer
+ships and installs its own copy of R.)

src/sage/features/standard.py

@@ -33,5 +33,6 @@ def all_features():
             PythonModule('ptyprocess', spkg='ptyprocess', type='standard'),
             PythonModule('pyparsing', spkg='pyparsing', type='standard'),
             PythonModule('requests', spkg='requests', type='standard'),
+            PythonModule('rpy2', spkg='rpy2', type='standard'),
             PythonModule('scipy', spkg='scipy', type='standard'),
             PythonModule('sympy', spkg='sympy', type='standard')]