gh-114271: Make `_thread.ThreadHandle` thread-safe in free-threaded builds (GH-115190)

Make `_thread.ThreadHandle` thread-safe in free-threaded builds

We protect the mutable state of `ThreadHandle` using a `_PyOnceFlag`.
Concurrent operations (i.e. `join` or `detach`) on `ThreadHandle` block
until it is their turn to execute or an earlier operation succeeds.
Once an operation has been applied successfully all future operations
complete immediately.

The `join()` method is now idempotent. It may be called multiple times
but the underlying OS thread will only be joined once. After `join()`
succeeds, any future calls to `join()` will succeed immediately.

The internal thread handle `detach()` method has been removed.