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: Ifd61bc8e043ac9af446ae31bffd82f67f34facbc

monasca_api/cmd/monasca_db.py

@@ -16,8 +16,6 @@
    CLI interface for monasca database management.
 """
 
-from __future__ import print_function
-
 from oslo_config import cfg
 from oslo_db.sqlalchemy.migration_cli.ext_alembic import AlembicExtension
 

monasca_api/common/repositories/sqla/models.py

@@ -15,11 +15,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
 from sqlalchemy import Column
 from sqlalchemy.ext import compiler
 from sqlalchemy.sql import expression

monasca_api/db/alembic/env.py

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