gh-123471: Make itertools.zip_longest safe in the FT build

[eendebakpt]

ziplongest_next is not thread safe due to setting the iterators in lz->ittuple to NULL on exhaustion. In addition, the lz->numactive is mutable state.

The usual approach to making the iterators in this module thread-safe by not setting the underlying iterator to NULL is not working here, since the NULL value is used as a signal (e.g. the check it == NULL). We therefore make the iterator safe using a critical section.

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