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

[eendebakpt]

We use a critical section similar to the approach in #132814.

Note: the accumulate has mutable internal state lz->initial and lz->total. With some refactoring we can replace the lz->init pointer with a C int (where atomics can be used), but the lz->total remains a pointer that is mutated on each iteration.

A side benefit of the refactoring would be that a decref in accumulate_dealloc is avoided and visit in accumulate_traverse. I decided not to include the refactoring in this PR (it would involve some churn), but if a core dev feels otherwise I will include it.

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