gh-115999: Add free-threaded specialization for FOR_ITER (#128798)

gh-115999: Add free-threaded specialization for FOR_ITER (#128798)

Add free-threaded versions of existing specialization for FOR_ITER (list, tuples, fast range iterators and generators), without significantly affecting their thread-safety. (Iterating over shared lists/tuples/ranges should be fine like before. Reusing iterators between threads is not fine, like before. Sharing generators between threads is a recipe for significant crashes, like before.)