gh-112069: Make sets thread-safe with the GIL disabled (#113800)

This makes nearly all the operations on set thread-safe in the free-threaded build, with the exception of `_PySet_NextEntry` and `setiter_iternext`.

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>