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

[eendebakpt]

We make itertools.islice safe under free-threading (e.g. guard against crashes) by:

• Atomically getting and setting the mutable fields cnt and next
• Using the step field as a sentinel for iterator exhaustion
• Not clearing the underlying iterator upon exhaustion

Note: several other iterators in the itertools module have been made safe using a critical section (e.g. #144402). That approach works here as well and would make the approach a bit more uniform and more readable. Downside of a critical section it that is scales less well.

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