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: I15d451f6a832b2e698b28e7351a36ef7aea92abe

detail.py

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

openstack_requirements/cmds/check_conflicts.py

@@ -14,8 +14,6 @@
 
 """
 
-from __future__ import print_function
-
 import argparse
 import sys
 import traceback

openstack_requirements/cmds/check_exists.py

@@ -15,8 +15,6 @@ u-c.
 
 """
 
-from __future__ import print_function
-
 import argparse
 
 from packaging.specifiers import SpecifierSet

openstack_requirements/cmds/check_py2.py

@@ -12,8 +12,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-from __future__ import print_function
-
 import argparse
 
 import pkg_resources

openstack_requirements/cmds/generate.py

@@ -11,8 +11,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import print_function
-
 import copy
 import hashlib
 import optparse

openstack_requirements/cmds/normalize_requirements.py

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

openstack_requirements/tests/test_check_constraints.py

@@ -9,7 +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 print_function
 
 import io
 import mock

openstack_requirements/tests/test_update.py

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

openstack_requirements/tests/test_update_pbr.py

@@ -16,8 +16,6 @@
 
 # Based on test_update.py
 
-from __future__ import print_function
-
 import testtools
 
 from openstack_requirements.cmds import update

openstack_requirements/tests/test_update_suffix.py

@@ -12,8 +12,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from __future__ import print_function
-
 import testtools
 
 from openstack_requirements.cmds import update

tools/check-install.py

@@ -1,7 +1,5 @@
 #!/usr/bin/env python
 
-from __future__ import print_function
-
 import importlib
 import re
 import sys