Log Message:
Update devel/py-futures to 3.1.1.

- 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