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This change facilitates the creation of a new software release for
Python 3.

futures is a backport of Py3's concurrent.futures module.

By upgrading gunicorn to version 19.10.0 (which has:

if sys.version_info[0] < 3:
    extra_require['gthread'] = ['futures']

), we won't need to remove the "futures" egg from software-py3.cfg,
i.e.:

eggs -=
  futures