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: If01e22f2c01fdde9fc3569c243e805207b9cad88

oslo_i18n/tests/test_message.py

@@ -14,8 +14,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from __future__ import unicode_literals
-
 import logging
 from unittest import mock
 import warnings

oslo_i18n/tests/test_translate.py

@@ -14,8 +14,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from __future__ import unicode_literals
-
 from unittest import mock
 
 from oslotest import base as test_base