Update futures to 3.4.0

[pyup-bot]

This PR updates futures from 3.1.1 to 3.4.0.

Changelog

3.4.0

=====

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

3.3.0

=====

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

3.2.0

=====

- 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

Links

• PyPI: https://pypi.org/project/futures
• Changelog: https://pyup.io/changelogs/futures/
• Repo: https://github.com/agronholm/pythonfutures
• Docs: https://pythonhosted.org/futures/