…ython#109810)

concurrent.futures: The *executor manager thread* now catches
exceptions when adding an item to the *call queue*. During Python
finalization, creating a new thread can now raise RuntimeError. Catch
the exception and call terminate_broken() in this case.

Add test_python_finalization_error() to test_concurrent_futures.

concurrent.futures._ExecutorManagerThread changes:

* terminate_broken() no longer calls shutdown_workers() since the
  call queue is no longer working anymore (read and write ends of
  the queue pipe are closed).
* terminate_broken() now terminates child processes, not only
  wait until they complete.
* _ExecutorManagerThread.terminate_broken() now holds shutdown_lock
  to prevent race conditons with ProcessPoolExecutor.submit().

multiprocessing.Queue changes:

* Add _terminate_broken() method.
* _start_thread() sets _thread to None on exception to prevent
  leaking "dangling threads" even if the thread was not started
  yet.

(cherry picked from commit 6351842)