Issue #15646: Prevent equivalent of a fork bomb when using multiprocessing

on Windows without the "if __name__ == '__main__'" idiom.

Lib/test/mp_fork_bomb.py

@@ -0,0 +1,13 @@
+import multiprocessing, sys
+
+def foo():
+    print("123")
+
+# Because "if __name__ == '__main__'" is missing this will not work
+# correctly on Windows.  However, we should get a RuntimeError rather
+# than the Windows equivalent of a fork bomb.
+
+p = multiprocessing.Process(target=foo)
+p.start()
+p.join()
+sys.exit(p.exitcode)