gh-123471: Make concurrent iteration over itertools.pairwise thread-safe

[eendebakpt]

We use a critical section similar to the approach in #132814. The ideas from an earlier PR #125417 are too complex (or not sufficient for thread-safety).

We also avoid an incref/decref pair.

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

[rhettinger]

I'm thinking that this isn't the right strategy for itertools. Instead, we should implement the same semantics as for generators which is to raise an exception if a concurrent call occurs.

If a user wants concurrency for either generators or itertools, then they with need to wrap the iterators with serialize or a concurrent version of tee as discussed in the planning issue.

[eendebakpt]

@rhettinger I agree with you that concurrent iteration over an iterator from itertools is probably not what the user wants and having to wrap in serialize is a minor inconvenience to make concurrent iteration possible.

However, the change you propose is not backwards compatible. Do you want to start a discussion on DPO for this?