gh-114271: Make `_thread.lock` thread-safe in free-threaded builds (#116433)

Previously, the `locked` field was set after releasing the lock. This reverses
the order so that the `locked` field is set while the lock is still held.

There is still one thread-safety issue where `locked` is checked prior to
releasing the lock, however, in practice that will only be an issue when
unlocking the lock is contended, which should be rare.