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: I2f9d2114b2c5eb66f241646f1896ea17a160e3f3

keystone/cmd/cli.py

@@ -12,9 +12,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from __future__ import absolute_import
-from __future__ import print_function
-
 import argparse
 import datetime
 import os

keystone/cmd/doctor/ldap.py

@@ -10,8 +10,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from __future__ import print_function
-
 import os
 import re
 

keystone/identity/backends/ldap/core.py

@@ -11,7 +11,6 @@
 # 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 absolute_import
 import uuid
 
 import ldap.filter

keystone/oauth1/core.py

@@ -14,8 +14,6 @@
 
 """Main entry point into the OAuth1 service."""
 
-from __future__ import absolute_import
-
 import uuid
 
 import oauthlib.common

keystone/server/flask/application.py

@@ -10,8 +10,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from __future__ import absolute_import
-
 import functools
 import sys
 

keystone/tests/unit/core.py

@@ -11,8 +11,6 @@
 # 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 absolute_import
 import atexit
 import base64
 import contextlib