Remove self-cimports

[mkoeppe]

Some Cython modules contain redundant cimport statements that cimport from themselves - directly or indirectly.

We remove them and make some related cleanups of imports/cimports.

This helps with Cython 3.0.2, in which the self-cimports trigger surprising behavior in the presence of implicit namespace packages.

• Cherry-picked from Modularization of sagelib: Distributions sagemath-{brial,combinat,eclib,flint,gap,giac,glpk,graphs,groups,homfly,lcalc,libbraiding,libecm,linbox,modules,mpmath,ntl,pari,plot,polyhedra,schemes,singular,standard-no-symbolics,symbolics} #35095

📝 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 1ba481b; changes) is ready! 🎉

[fchapoton]

looks ok

[fchapoton]

maybe the changes in src/sage/graphs/comparability.pyx are causing problems ? we see some failures there in our broken CI right now