gh-125451: Fix deadlock in ProcessPoolExecutor shutdown (#125492)

There was a deadlock when `ProcessPoolExecutor` shuts down at the same
time that a queueing thread handles an error processing a task.

Don't use `_shutdown_lock` to protect the `_ThreadWakeup` pipes -- use
an internal lock instead. This fixes the ordering deadlock where the
`ExecutorManagerThread` holds the `_shutdown_lock` and joins the
queueing thread, while the queueing thread is attempting to acquire the
`_shutdown_lock` while closing the `_ThreadWakeup`.

Lib/test/test_concurrent_futures/test_shutdown.py

@@ -253,9 +253,6 @@ def test_cancel_futures_wait_false(self):
 
 
 class ProcessPoolShutdownTest(ExecutorShutdownTest):
-    # gh-125451: 'lock' cannot be serialized, the test is broken
-    # and hangs randomly
-    @unittest.skipIf(True, "broken test")
     def test_processes_terminate(self):
         def acquire_lock(lock):
             lock.acquire()