Draft: gh-123471: Make concurrent iteration over itertools.pairwise safe under free-threading

[eendebakpt]

We make concurrent iteration over itertools.pairwise safe by:

• Using _PyObject_IsUniquelyReferenced for re-use of the result type
• Holding a reference to po->it instead of a borrowed reference
• Obtaining a reference to po->old early.

Also see #123848

To make pairwise_next safe we need to prevent concurrent iterators from accessing the cleared po->it

• Idea 1: In the first line of pairwise_next replace PyObject *it = po->it; with PyObject *it = Py_XInRef(po->it);.
  Then every thread will have a true reference to the iterator, and not a borrowed reference.
  Simple, but overhead is an incref/decref in the common case
• Idea 2a: Do not clear po->it. We then need another way to signal that the iterator has been exhausted.
  Use another variable "int iterator_exhausted" in the pairwise object. Requires a bit more memory
  for each pairwise object, but the check is a simple int comparison.
• Idea 2b: Do not clear po->it. We then need another way to signal that the iterator has been exhausted.
  Use po->old to set a sentinel. Requires a global sentinel in the module (or a sentinel per thread).
  In the top of pairwise_next we would have

PyObject *old = Py_XNewRef(po->old);
if (old == sentinel) {
      // iterator was exhausted earlier, return NULL
      Py_DECREF(old)
      return NULL;
}
if (old == NULL) {
      // first call to pairwise_next, normal code
      ...
}

An issue with this approach is that at the end of pairwise_next there is the line: Py_XSETREF(po->old, new);
So one thread can set the sentinel, but in another thread the value could be overwritten.
Maybe this is ok: this will not cause crashes, and we do not have to guarantee correct behaviour for
the free-threading build.
This approach does not require additional increfs/decrefs, but it does require setting up the sentinel.

In this PR we work out the first option.

• Issue: Make concurrent iteration over pairwise, combinations, permutations, cwr, product from itertools safe under free-threading #123471

[rhettinger]

Can I suggest that you start with enumerate() rather than pairwise(). The former is a built-in, so more people care about it and your edit will get more attention, feedback, and buy-in.

Also, I would like to keep the latter as simple as possible for now. I'm evaluating a proposal to add prefix and suffix options to pairwise(). This would be much harder to do after these edits.

[eendebakpt]

Can I suggest that you start with enumerate() rather than pairwise(). The former is a built-in, so more people care about it and

Sure! I created a PR for enumerate at #125734. For reversed there was an earlier PR #120971 where your review was requested. Would you have time to review that one?