MTN backport some changes from cpython

[tomMoral]

There has been a couple of bug fix and refactor on cpython concurrent.futures that we should backport in loky. In particular:

• bpo-39104: Fix hanging ProcessPoolExecutor on shutdown nowait with pickling failure python/cpython#17670 and follow up bpo-39995: Fix concurrent.futures _ThreadWakeup python/cpython#19760 || bpo-39995: CLN remove some locks in ProcessPoolExecutor python/cpython#19788 that fixes wait=False
• bpo-39678 RFC refactor queue manager thread python/cpython#18551 that refactored the thread_manager_executor.

I will try to do this ASAP.

[ogrisel]

@pierreglaser we should probably wait for this before releasing loky 2.7.0.