sage.combinat: Split some Cython modules (modularization fixes)

[mkoeppe]

📚 Description

Some basic modules of sage.combinat (.permutation, .combination, etc.) are needed in the distribution sagemath-categories (as of #35095). We make them importable (by splitting a Cython module into several parts and by using lazy imports) and separately testable (using # optional doctest directives).

Likewise, split the parts of sage.combinat.posets.hasse_cython that need the FLINT library out as a separate module (this is for the package sagemath-graphs in #35095)

• Split out from sage.misc.misc, sage.combinat: Modularization fixes #35564
• Part of Meta-ticket: Modularize sagelib into separate distributions (pip-installable packages) sagemath-... for Sage 10.x #29705

📝 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

[mkoeppe]

@dcoudert, from #35564 (comment):

Shouldn't you add # cython: binding=True ?

I don't know; the source file from which I split it out did not have this directive if I'm not mistaken.

I'll be happy to add it to if you recommend it?

[dcoudert]

@dcoudert, from #35564 (comment):

Shouldn't you add # cython: binding=True ?

I don't know; the source file from which I split it out did not have this directive if I'm not mistaken.

I'll be happy to add it to if you recommend it?

I had to add # cython: binding=True in some .pyx files in order to import some methods from the console, e.g., with from sage.graphs.graph_decompositions.cutwidth import cutwidth. This is convenient in some cases.

[mkoeppe]

OK, done in fe07439.

This will be the default anyway in the upcoming Cython 3 (#29863).

[github-actions]

Documentation preview for this PR (built with commit fe07439) is ready! 🎉

[dcoudert]

LGTM.

[mkoeppe]

Thanks a lot for reviewing!