get rid of one ParentWithGens

[fchapoton]

mirco-ticket to get rid of one ParentWithGens

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.

[github-actions]

Documentation preview for this PR (built with commit 086a13f; changes) is ready! 🎉

[tscrim]

Goodbye, you will not be missed.

However, I think we should be a little careful here. Have you checked that the compiled C code just does a C-level redirection? Otherwise we might be some speed regressions.

[fchapoton]

Indeed, I am probably not careful enough. I am not able to check anything about the generated code, sorry.

[tscrim]

I checked and it should be the same as before:

      __pyx_t_1 = __Pyx_GetItemInt(__pyx_v_self->__pyx_base.__pyx_base.__pyx_base.__pyx_base.__pyx_base._parent->__pyx_base._names, 0, long, 1, __Pyx_PyInt_From_long, 0, 0, 1); if (unlikely(!__pyx_t_1)) __PYX_ERR(0, 2342, __pyx_L1_error)
      __Pyx_GOTREF(__pyx_t_1);
      __pyx_t_6 = PyObject_Hash(__pyx_t_1); if (unlikely(__pyx_t_6 == ((Py_hash_t)-1))) __PYX_ERR(0, 2342, __pyx_L1_error)
      __Pyx_DECREF(__pyx_t_1); __pyx_t_1 = 0;
      __pyx_v_var_name_hash = __pyx_t_6;

versus before

      __pyx_t_1 = __Pyx_GetItemInt(((struct __pyx_obj_4sage_9structure_11parent_gens_ParentWithGens *)__pyx_v_self->__pyx_base.__pyx_base.__pyx_base.__pyx_base.__pyx_base._parent)->__pyx_base.__pyx_base.__pyx_base.__pyx_base._names, 0, long, 1, __Pyx_PyInt_From_long, 0, 0, 1); if (unlikely(!__pyx_t_1)) __PYX_ERR(0, 2343, __pyx_L1_error)
      __Pyx_GOTREF(__pyx_t_1);
      __pyx_t_4 = PyObject_Hash(__pyx_t_1); if (unlikely(__pyx_t_4 == ((Py_hash_t)-1))) __PYX_ERR(0, 2343, __pyx_L1_error)
      __Pyx_DECREF(__pyx_t_1); __pyx_t_1 = 0;
      __pyx_v_var_name_hash = __pyx_t_4;

So positive review.