move single_valued method of MZV to auxiliary F ring

[fchapoton]

📚 Description

This is moving a little piece of code from MZV to a more proper place (auxiliary ring).

📝 Checklist

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

⌛ Dependencies

[codecov-commenter]

Codecov Report

Base: 88.59% // Head: 88.59% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (7a9d18e) compared to base (fbb4127).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 7a9d18e differs from pull request most recent head 875738a. Consider uploading reports for the commit 875738a to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #35086    +/-   ##
=========================================
  Coverage    88.59%   88.59%            
=========================================
  Files         2140     2136     -4     
  Lines       396961   396143   -818     
=========================================
- Hits        351677   350975   -702     
+ Misses       45284    45168   -116     

Impacted Files	Coverage Δ
src/sage/modular/multiple_zeta.py	94.68% <100.00%> (-0.04%)	⬇️
src/sage/modular/multiple_zeta_F_algebra.py	94.93% <100.00%> (+0.16%)	⬆️
src/sage/misc/banner.py	72.97% <0.00%> (-14.87%)	⬇️
src/sage/categories/posets.py	98.76% <0.00%> (-1.24%)	⬇️
src/sage/modular/hecke/algebra.py	94.65% <0.00%> (-1.07%)	⬇️
src/sage/combinat/schubert_polynomial.py	91.66% <0.00%> (-0.65%)	⬇️
...e/combinat/cluster_algebra_quiver/mutation_type.py	52.37% <0.00%> (-0.60%)	⬇️
src/sage/sets/integer_range.py	91.41% <0.00%> (-0.51%)	⬇️
src/sage/groups/matrix_gps/coxeter_group.py	96.07% <0.00%> (-0.50%)	⬇️
src/sage/rings/polynomial/ore_polynomial_ring.py	89.40% <0.00%> (-0.43%)	⬇️
... and 61 more

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[videlec]

Could you be more explicit in the documentation about the mathematical meaning of single-valued version of self?

[videlec]

why no space around * and +?

[fchapoton]

wtf ? I have first pulled develop, and now this is a mess..

[videlec]

Looks good to me.

I did not understand the purpose of merging develop.

[fchapoton]

This was my workflow, when develop was not a moving target. I was always working on top of the latest develop. I guess I have to adapt, but it's not clear to me how.

[videlec]

if you want to do that you would better do a rebase rather than a merge.

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 875738a

[fchapoton]

would you please "approve" the changes ?