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: I8bad429563b89ad459d1b1910083484f2ccf9fff

cinder/cmd/manage.py

@@ -50,8 +50,6 @@
 
 """CLI interface for cinder management."""
 
-from __future__ import print_function
-
 try:
     import collections.abc as collections
 except ImportError:

cinder/image/glance.py

@@ -16,9 +16,6 @@
 
 """Implementation of an image service that uses Glance as the backend"""
 
-
-from __future__ import absolute_import
-
 import copy
 import itertools
 import random

cinder/tests/fixtures.py

@@ -15,8 +15,6 @@
 #    under the License.
 
 """Fixtures for Cinder tests."""
-# NOTE(mriedem): This is needed for importing from fixtures.
-from __future__ import absolute_import
 
 import logging as std_logging
 import os

cinder/volume/drivers/dell_emc/unity/utils.py

@@ -13,8 +13,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from __future__ import division
-
 import contextlib
 from distutils import version
 import functools

cinder/volume/drivers/storpool.py

@@ -15,8 +15,6 @@
 
 """StorPool block device driver"""
 
-from __future__ import absolute_import
-
 import platform
 
 from oslo_config import cfg