cylint: remove unused imports in folders [s-z]*

[fchapoton]

📚 Description

remove unused imports found by cython-lint in all folders from s* to z*

📝 Checklist

• The title is concise, informative, and self-explanatory.
• 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

[tscrim]

Someone did a bad import:

  File "/__w/sage/sage/src/sage/structure/formal_sum.py", line 76, in <module>
    from sage.structure.coerce import LeftModuleAction, RightModuleAction

This is causing the build to fail. :3

[tscrim]

LGTM assuming all tests pass. If they do, then set a positive review.

[github-actions]

Documentation preview for this PR (built with commit f8d3608) is ready! 🎉

[fchapoton]

thanks, Travis. Now setting to positive. Maybe you should approve the code, as I have squashed the branch.

[tscrim]

The approval seem to carry over regardless if there are further changes pushed... Anyways, better to reapprove than to have this bitrot.