gh-157176: Fix GC tracking in PyStructSequence_New

[ashm-dev]

• Issue: Memory leak on interpreter shutdown when reference cycle exists between structseq type and its instance #157176

[ashm-dev]

@sergey-miryanov, could you please review this PR?

[sergey-miryanov]

This looks safe, but we usually try to avoid exposing partially constructed objects to the GC. I'm not sure I get why this change fixes your repro. Could you explain a bit?

[ashm-dev]

• Why it fixes the repro: time.gmtime() is not tracked by the GC. Because of that, the GC cannot see the cycle type(t) -> t -> type(t), so the entire heap type leaks at shutdown. Tracking t lets GC traverse t -> Py_TYPE(t) and collect the cycle.

• Partially constructed object: All items are pre-initialized to NULL. Both structseq_traverse (Py_VISIT) and structseq_dealloc (Py_XDECREF) safely ignore NULL. This follows PyTuple_New(), which also tracks the object with NULL items since C API callers have no separate "finish" step.

[sergey-miryanov]

So, this change affects all call sites where PyStructSequence_New is called without a subsequent call to _PyObject_GC_TRACK. But at first glance, many of them don't seem to need tracking for PyStructSequence_New instances.

@vstinner Do we need frozen version of PyStructSequence?