bpo-39776: Lock ++interp->tstate_next_unique_id. (GH-18746) (#18746) (#18752)

- Threads created by PyGILState_Ensure() could have a duplicate tstate->id.

(cherry picked from commit b3b9ade4a3d3fe00d933bcd8fc5c5c755d1024f9)
diff --git a/Python/pystate.c b/Python/pystate.c
index aba673c..9f99060 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -606,13 +606,12 @@
     tstate->context = NULL;
     tstate->context_ver = 1;
 
-    tstate->id = ++interp->tstate_next_unique_id;
-
     if (init) {
         _PyThreadState_Init(runtime, tstate);
     }
 
     HEAD_LOCK(runtime);
+    tstate->id = ++interp->tstate_next_unique_id;
     tstate->prev = NULL;
     tstate->next = interp->tstate_head;
     if (tstate->next)