[2.7] bpo-24484: Avoid race condition in multiprocessing cleanup (GH-2159) (#2168)

* bpo-24484: Avoid race condition in multiprocessing cleanup

The finalizer registry can be mutated while inspected by multiprocessing
at process exit.

* Use test.support.start_threads()

* Add Misc/NEWS.
(cherry picked from commit 1eb6c0074d17f4fd425cacfdda893d65f5f77f0a)
2 files changed