gh-116522: Refactor _PyThreadState_DeleteExcept (#117131)

Split `_PyThreadState_DeleteExcept` into two functions:

- `_PyThreadState_RemoveExcept` removes all thread states other than one
  passed as an argument. It returns the removed thread states as a
  linked list.

- `_PyThreadState_DeleteList` deletes those dead thread states. It may
  call destructors, so we want to "start the world" before calling
  `_PyThreadState_DeleteList` to avoid potential deadlocks.

Python/ceval_gil.c

@@ -579,9 +579,8 @@ PyEval_ReleaseThread(PyThreadState *tstate)
 }
 
 #ifdef HAVE_FORK
-/* This function is called from PyOS_AfterFork_Child to destroy all threads
-   which are not running in the child process, and clear internal locks
-   which might be held by those threads. */
+/* This function is called from PyOS_AfterFork_Child to re-initialize the
+   GIL and pending calls lock. */
 PyStatus
 _PyEval_ReInitThreads(PyThreadState *tstate)
 {
@@ -598,8 +597,6 @@ _PyEval_ReInitThreads(PyThreadState *tstate)
     struct _pending_calls *pending = &tstate->interp->ceval.pending;
     _PyMutex_at_fork_reinit(&pending->mutex);
 
-    /* Destroy all threads except the current one */
-    _PyThreadState_DeleteExcept(tstate);
     return _PyStatus_OK();
 }
 #endif