gh-104690 Disallow thread creation and fork at interpreter finalization (#104826)

Disallow thread creation and fork at interpreter finalization.

in the following functions, check if interpreter is finalizing and raise `RuntimeError` with appropriate message:
* `_thread.start_new_thread` and thus `threading`
* `posix.fork`
* `posix.fork1`
* `posix.forkpty`
* `_posixsubprocess.fork_exec` when a `preexec_fn=` is supplied.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/test/test_os.py

@@ -4706,6 +4706,22 @@ class ForkTests(unittest.TestCase):
         self.assertEqual(err.decode("utf-8"), "")
         self.assertEqual(out.decode("utf-8"), "")
 
+    def test_fork_at_exit(self):
+        code = """if 1:
+            import atexit
+            import os
+
+            def exit_handler():
+                pid = os.fork()
+                if pid != 0:
+                    print("shouldn't be printed")
+
+            atexit.register(exit_handler)
+        """
+        _, out, err = assert_python_ok("-c", code)
+        self.assertEqual(b"", out)
+        self.assertIn(b"can't fork at interpreter shutdown", err)
+
 
 # Only test if the C version is provided, otherwise TestPEP519 already tested
 # the pure Python implementation.