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 +1,16 @@
+import multiprocessing
+
+def foo(conn):
+    conn.send("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.
+
+r, w = multiprocessing.Pipe(False)
+p = multiprocessing.Process(target=foo, args=(w,))
+p.start()
+w.close()
+print(r.recv())
+r.close()
+p.join()