more fixes for superfluous cython imports

[fchapoton]

📚 Description

found by cython-lint ; removing a few more useless cython imports

📝 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

[github-actions]

Documentation preview for this PR (built with commit 71aef78) is ready! 🎉

[dcoudert]

By curiosity, is it ok to call sig_on() but not sig_off() in src/sage/schemes/elliptic_curves/descent_two_isogeny.pyx ?

[fchapoton]

Aucune idée. Mais effectivement, on peut se poser la question. Parfois le sig_off est appelé dans une sous-routine.

[dcoudert]

Since this sig_on has been added by Jeroen Demeyer without the sig_off, I assume it has been done on purpose.