Use Cython 3 implicit namespace package support

[mkoeppe]

Making sage.misc.package_dir.cython_namespace_package_support a no-op for Cython 3.

Keeping the code for Cython 0.x around for use in downstream 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

• Depends on Remove self-cimports #36207 (merged here)
• Depends on Update Cython to 3.0.2 #36110 (merged here)
• Depends on Replace relative imports by absolute ones in structure #36573 (need to rebase on top)
• Depends on the changes in dfceb98, 7d04241, ef2f651 in 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
• Depends on Replace relative imports by absolute ones in sage.{matrix,matroids,modules,stats} #36594 (cherry-picked from above)
• Depends on Replace relative imports by absolute ones in sage.{coding,combinat,graphs} #36595 (cherry-picked from above)
• Depends on Replace relative imports by absolute ones in sage.{geometry,groups,numerical,plot} #36596 (cherry-picked from above)
• Depends on Replace relative imports by absolute ones in sage.libs.ntl #36605
• Depends on Replace relative imports by absolute ones in sage.libs except .ntl #36612 (cherry-picked from above)
• Depends on Replace relative imports by absolute ones in sage.{algebras,arith,categories,cpython,data_structures,misc,modular,rings,sat,symbolic} #36666 (cherry-picked from above, merged here)
• Depends on Replace relative imports by absolute ones in categories #36572 (dup)
• Depends on Replace relative imports by absolute ones in rings #36588 (dup)
• Depends on Replace relative imports by absolute ones in a few packages #36589 (dup)

[mkoeppe]

This change (disabling our sage.misc.package_dir.cython_namespace_package_support) leads to many errors
sage/algebras/quatalg/quaternion_algebra_cython.pyx:38:0: relative cimport from non-package directory is not allowed.

• This is related but not the same as [BUG] namespace packages as in pep 420 cannot be cimported cython/cython#5335 @tornaria

[mkoeppe]

https://github.com/sagemath/sage/actions/runs/6777529749/job/18421318603?pr=36228#step:9:13512

  building 'static_sparse_graph' extension
  /usr/share/miniconda3/envs/sage/bin/x86_64-conda-linux-gnu-cc -Wno-unused-result -Wsign-compare -DNDEBUG -fwrapv -O2 -Wall -fPIC -O2 -isystem /usr/share/miniconda3/envs/sage/include -fPIC -O2 -isystem /usr/share/miniconda3/envs/sage/include -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /usr/share/miniconda3/envs/sage/include -DNDEBUG -D_FORTIFY_SOURCE=2 -O2 -isystem /usr/share/miniconda3/envs/sage/include -fPIC -Isage/data_structures -I/usr/share/miniconda3/envs/sage/lib/python3.9/site-packages/cysignals -Isage/cpython -I/home/runner/work/sage/sage/src -I/usr/share/miniconda3/envs/sage/lib/python3.9/site-packages/numpy/core/include -I/usr/share/miniconda3/envs/sage/include/python3.9 -I/usr/share/miniconda3/envs/sage/include/python3.9 -c sage/graphs/base/static_sparse_graph.cpp -o /tmp/tmpc71e2zq9.build-temp/sage/graphs/base/static_sparse_graph.o
  sage/graphs/base/static_sparse_graph.cpp: In function 'PyObject* __pyx_pf_19static_sparse_graph_6spectral_radius(PyObject*, PyObject*, PyObject*)':
  sage/graphs/base/static_sparse_graph.cpp:20569:127: error: no match for 'operator=' (operand types are '__pyx_t_19static_sparse_graph_short_digraph_s' and '__pyx_t_4sage_6graphs_4base_19static_sparse_graph_short_digraph_s')
  20569 |   (__pyx_v_g[0]) = (((struct __pyx_obj_4sage_6graphs_4base_21static_sparse_backend_StaticSparseBackend *)__pyx_t_8)->_cg->g[0]);
        |                                                                                                                               ^
  sage/graphs/base/static_sparse_graph.cpp:1830:8: note: candidate: 'constexpr __pyx_t_19static_sparse_graph_short_digraph_s& __pyx_t_19static_sparse_graph_short_digraph_s::operator=(const __pyx_t_19static_sparse_graph_short_digraph_s&)'
   1830 | struct __pyx_t_19static_sparse_graph_short_digraph_s {
        |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  sage/graphs/base/static_sparse_graph.cpp:1830:8: note:   no known conversion for argument 1 from '__pyx_t_4sage_6graphs_4base_19static_sparse_graph_short_digraph_s' to 'const __pyx_t_19static_sparse_graph_short_digraph_s&'
  sage/graphs/base/static_sparse_graph.cpp:1830:8: note: candidate: 'constexpr __pyx_t_19static_sparse_graph_short_digraph_s& __pyx_t_19static_sparse_graph_short_digraph_s::operator=(__pyx_t_19static_sparse_graph_short_digraph_s&&)'