* bpo-31249: Fix ref cycle in ThreadPoolExecutor
concurrent.futures: WorkItem.run() used by ThreadPoolExecutor now
breaks a reference cycle between an exception object and the WorkItem
object. ThreadPoolExecutor.shutdown() now also clears its threads
set.
* shutdown() now only clears threads if wait is true.
* Revert changes on shutdown()
bpo-29212: Fix the ugly ThreadPoolExecutor thread name.
Fixes the newly introduced ugly default thread name for concurrent.futures
thread.ThreadPoolExecutor threads. They'll now resemble the old <=3.5
threading default Thread-x names by being named ThreadPoolExecutor-y_n.