Description of problem:
Boto 3 uses concurrent.futures in s3.transfer. Without the backport of concurrent.futures from python-futures installed, s3 cannot be used from Boto 3.

How reproducible:
Always

Steps to Reproduce:
1. Install python2-boto3 via dnf *without* python-futures previously installed
2. Configure AWS credentials per https://github.com/boto/boto3#quick-start
3. From a Python interpreter:

    >>> import boto3
    >>> s3 = boto3.resource('s3')

4. Manually install python-futures via dnf
5. Repeat snippet in Python interpreter; call no longer crashes.

Actual results:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/boto3/__init__.py", line 87, in resource
    return _get_default_session().resource(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/boto3/session.py", line 292, in resource
    aws_session_token=aws_session_token, config=config)
  File "/usr/lib/python2.7/site-packages/boto3/session.py", line 200, in client
    aws_session_token=aws_session_token, config=config)
  File "/usr/lib/python2.7/site-packages/botocore/session.py", line 796, in create_client
    client_config=config, api_version=api_version)
  File "/usr/lib/python2.7/site-packages/botocore/client.py", line 59, in create_client
    cls = self._create_client_class(service_name, service_model)
  File "/usr/lib/python2.7/site-packages/botocore/client.py", line 76, in _create_client_class
    base_classes=bases)
  File "/usr/lib/python2.7/site-packages/botocore/hooks.py", line 226, in emit
    return self._emit(event_name, kwargs)
  File "/usr/lib/python2.7/site-packages/botocore/hooks.py", line 209, in _emit
    response = handler(**kwargs)
  File "/usr/lib/python2.7/site-packages/boto3/utils.py", line 59, in _handler
    module = import_module(module)
  File "/usr/lib/python2.7/site-packages/boto3/utils.py", line 52, in import_module
    __import__(name)
  File "/usr/lib/python2.7/site-packages/boto3/s3/inject.py", line 13, in <module>
    from boto3.s3.transfer import S3Transfer
  File "/usr/lib/python2.7/site-packages/boto3/s3/transfer.py", line 135, in <module>
    from concurrent import futures
ImportError: No module named concurrent


Expected results:

python2-boto3 RPM requires python-futures so that python-futures also gets installed and provided snippet does not crash.


Additional info:
Usage of concurrent.futures in source code: https://github.com/boto/boto3/blob/develop/boto3/s3/transfer.py#L135
Boto 3 setup.cfg requiring futures: https://github.com/boto/boto3/blob/develop/setup.cfg#L8
Missing requires: http://pkgs.fedoraproject.org/cgit/rpms/python-boto3.git/tree/python-boto3.spec#n41

Impacts versions 22, 23, and rawhide, but Bugzilla will only let me select one.