Futures

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- Backported bpo-21423: Add an initializer argument to {Process,Thread}PoolExecutor
(only ThreadPoolExecutor for now; PR by Fahrzin Hemmati)

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- Backported bpo-24882: Let ThreadPoolExecutor reuse idle threads before creating new thread

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- The ThreadPoolExecutor class constructor now accepts an optional ``thread_name_prefix``
argument to make it possible to customize the names of the threads created by the pool.
Upstream contribution by Gregory P. Smith in https://bugs.python.org/issue27664.
- Backported fixes from upstream (thanks Lisandro Dalcin):

- python/cpython1560
- python/cpython3270
- python/cpython3830

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- Backported sanity checks for the ``max_workers`` constructor argument for
ThreadPoolExecutor and ProcessPoolExecutor
- Set the default value of ``max_workers`` in ThreadPoolExecutor to ``None``,
as in upstream code (computes the thread pool size based on the number of
CPUs)
- Added support for old-style exception objects
- Switched to the PSF license

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- (Failed release)

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- Fixed OverflowError with ProcessPoolExecutor on Windows (regression
introduced in 3.0.4)