[3.13] gh-117657: Fix data races reported by TSAN in some set methods (GH-120914) (#121240)

Refactor the fast Unicode hash check into `_PyObject_HashFast` and use relaxed
atomic loads in the free-threaded build.

After this change, the TSAN doesn't report data races for this method.
(cherry picked from commit 294e72496439da984cb8dba9100d3613c8cc8a6d)

Co-authored-by: AN Long <aisk@users.noreply.github.com>