gh-116946: add Py_TPFLAGS_IMMUTABLETYPE to select.[e]poll

[picnixz]

I'm not sure about kqueue as I can't check this myself. However note the following leak on main:

$ ./python -X showrefcount -c 'import select; s = select.poll(); t = type(s); t._do = s'
[74 refs, 43 blocks]

Instead of implementing the full GC protocol, we can make the types immutable.

Rationale for not backporting this: #138341 (comment).

• Issue: Heap types implemented in C must have Py_TPFLAGS_HAVE_GC and implement tp_traverse #116946

[vstinner]

LGTM.

I don't know if "cannot create cycles as it only contains exact ints" comment is correct or not.

[picnixz]

I don't know if "cannot create cycles as it only contains exact ints" comment is correct or not.

That's why I actually asked. The ints are constructed internally (AFAIU) so I that's why I didn't visit the dictionary. But I can visit it just to be sure.