[3.12] gh-113964: Don't prevent new threads until all non-daemon threads exit (GH-116677) (#117029)

Starting in Python 3.12, we prevented calling fork() and starting new threads
during interpreter finalization (shutdown). This has led to a number of
regressions and flaky tests. We should not prevent starting new threads
(or `fork()`) until all non-daemon threads exit and finalization starts in
earnest.

This changes the checks to use `_PyInterpreterState_GetFinalizing(interp)`,
which is set immediately before terminating non-daemon threads.

(cherry picked from commit 60e105c1c1)

Modules/_threadmodule.c

@@ -1190,7 +1190,7 @@ thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
                         "thread is not supported for isolated subinterpreters");
         return NULL;
     }
-    if (interp->finalizing) {
+    if (_PyInterpreterState_GetFinalizing(interp) != NULL) {
         PyErr_SetString(PyExc_RuntimeError,
                         "can't create new thread at interpreter shutdown");
         return NULL;