gh-116946: fully implement GC protocol for 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]

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