Stop to use the __future__ module.

The __future__ module [1] was used in this context to ensure compatibility
between python 2 and python 3.

We previously dropped the support of python 2.7 [2] and now we only support
python 3 so we don't need to continue to use this module and the imports
listed below.

Imports commonly used and their related PEPs:
- `division` is related to PEP 238 [3]
- `print_function` is related to PEP 3105 [4]
- `unicode_literals` is related to PEP 3112 [5]
- `with_statement` is related to PEP 343 [6]
- `absolute_import` is related to PEP 328 [7]

[1] https://docs.python.org/3/library/__future__.html
[2] https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html
[3] https://www.python.org/dev/peps/pep-0238
[4] https://www.python.org/dev/peps/pep-3105
[5] https://www.python.org/dev/peps/pep-3112
[6] https://www.python.org/dev/peps/pep-0343
[7] https://www.python.org/dev/peps/pep-0328

Change-Id: Ifeaa5ba961c849a40407f4b3cc172f1423df3eaf

magnumclient/common/cliutils.py

@@ -16,8 +16,6 @@
 # W0621: Redefining name %s from outer scope
 # pylint: disable=W0603,W0621
 
-from __future__ import print_function
-
 import getpass
 import inspect
 import os

magnumclient/shell.py

@@ -21,8 +21,6 @@
 """
 Command-line interface to the OpenStack Magnum API.
 """
-
-from __future__ import print_function
 import argparse
 import logging
 import os