sage.manifolds, sage.tensor: Add # optional doctest tags for modularization

[mkoeppe]

📚 Description

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 Meta-ticket: Modularize sagelib into separate distributions (pip-installable packages) sagemath-... for Sage 10.x #29705
• Split out from Meta-PR: More # optional doctest tags #34998

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

This is just two commits on top of:

• sage.features: Add sage.libs.singular, features for standard Python packages #35237

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 2c7ad61

[codecov-commenter]

Codecov Report

Patch coverage: 98.07% and project coverage change: +0.01 🎉

Comparison is base (52a81cb) 88.57% compared to head (2c7ad61) 88.58%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35267      +/-   ##
===========================================
+ Coverage    88.57%   88.58%   +0.01%     
===========================================
  Files         2140     2141       +1     
  Lines       397273   397416     +143     
===========================================
+ Hits        351891   352061     +170     
+ Misses       45382    45355      -27     

Impacted Files	Coverage Δ
src/sage/features/__init__.py	95.18% <ø> (ø)
src/sage/features/all.py	100.00% <ø> (ø)
src/sage/features/bliss.py	100.00% <ø> (ø)
src/sage/features/cddlib.py	100.00% <ø> (ø)
src/sage/features/csdp.py	40.74% <ø> (ø)
src/sage/features/cython.py	100.00% <ø> (ø)
src/sage/features/databases.py	96.15% <ø> (-0.15%)	⬇️
src/sage/features/dvipng.py	100.00% <ø> (ø)
src/sage/features/ffmpeg.py	22.58% <ø> (-2.42%)	⬇️
src/sage/features/gfan.py	100.00% <ø> (ø)
... and 83 more

... and 26 files with indirect coverage changes

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[egourgoulhon]

LGTM for sage.manifolds and sage.tensor parts.

[mkoeppe]

Thank you!

[vbraun]

sage -t --long --warn-long 73.8 --random-seed=123 src/sage/interfaces/r.py  # 3 doctests failed