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: Id785793c36b3a6819a7522525252c3fef15ebe2b

cinderclient/client.py

@@ -16,8 +16,6 @@
 #    under the License.
 """OpenStack Client interface. Handles the REST calls and responses."""
 
-from __future__ import print_function
-
 import glob
 import hashlib
 import imp

cinderclient/shell.py

@@ -17,8 +17,6 @@
 Command-line interface to the OpenStack Cinder API.
 """
 
-from __future__ import print_function
-
 import argparse
 import collections
 import getpass

cinderclient/shell_utils.py

@@ -12,8 +12,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from __future__ import print_function
-
 import sys
 import time
 

cinderclient/tests/unit/fakes.py

@@ -19,8 +19,6 @@ wrong the tests might raise AssertionError. I've indicated in comments the
 places where actual behavior differs from the spec.
 """
 
-from __future__ import print_function
-
 
 def assert_has_keys(dict, required=None, optional=None):
     required = required or []

cinderclient/utils.py

@@ -12,8 +12,6 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
-
-from __future__ import print_function
 import collections
 
 import os

cinderclient/v2/shell.py

@@ -14,8 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import print_function
-
 import argparse
 import collections
 import copy

cinderclient/v3/shell.py

@@ -14,8 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import print_function
-
 import argparse
 import collections
 import os

tools/install_venv_common.py

@@ -22,8 +22,6 @@ environment, it should be kept strictly compatible with Python 2.6.
 Synced in from openstack-common
 """
 
-from __future__ import print_function
-
 import optparse
 import os
 import subprocess