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: I0173b210e343ccf6b00c3b66959c001fdb1d699b

keystoneclient/tests/unit/test_ec2utils.py

@@ -12,8 +12,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from __future__ import unicode_literals
-
 import testtools
 
 from keystoneclient.contrib.ec2 import utils

keystoneclient/tests/unit/v2_0/client_fixtures.py

@@ -9,8 +9,6 @@
 #    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 unicode_literals
 import uuid
 
 from keystoneauth1 import fixture

keystoneclient/tests/unit/v3/client_fixtures.py

@@ -9,8 +9,6 @@
 #    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 unicode_literals
 import uuid
 
 from keystoneauth1 import fixture

keystoneclient/v3/contrib/oauth1/access_tokens.py

@@ -11,8 +11,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import unicode_literals
-
 from keystoneauth1 import plugin
 
 from keystoneclient import base

keystoneclient/v3/contrib/oauth1/request_tokens.py

@@ -11,8 +11,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import unicode_literals
-
 from keystoneauth1 import plugin
 from six.moves.urllib import parse as urlparse