[3.13] gh-120973: Fix thread-safety issues with threading.local (GH-121655) (#122042)

This is a small refactoring to the current design that allows us to
avoid manually iterating over threads.

This should also fix gh-118490.
(cherry picked from commit e059aa6)

Co-authored-by: mpage <[email protected]>

Author: miss-islington

Include/cpython/pystate.h

@@ -192,6 +192,14 @@ struct _ts {
     PyObject *previous_executor;
 
     uint64_t dict_global_version;
+
+    /* Used to store/retrieve `threading.local` keys/values for this thread */
+    PyObject *threading_local_key;
+
+    /* Used by `threading.local`s to be remove keys/values for dying threads.
+       The PyThreadObject must hold the only reference to this value.
+    */
+    PyObject *threading_local_sentinel;
 };
 
 #ifdef Py_DEBUG