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: I5d8e1d2320d75340fb802b4ef6a4f2a8fcafc8cd

charmhelpers/__init__.py

@@ -14,8 +14,6 @@
 
 # Bootstrap charm-helpers, installing its dependencies if necessary using
 # only standard libraries.
-from __future__ import print_function
-from __future__ import absolute_import
 
 import functools
 import inspect

charmhelpers/contrib/hardening/audits/apt.py

@@ -11,8 +11,6 @@
 # WITHOUT 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  # required for external apt import
 from six import string_types
 
 from charmhelpers.fetch import (

charmhelpers/contrib/python.py

@@ -12,8 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import absolute_import
-
 # deprecated aliases for backwards compatibility
 from charmhelpers.fetch.python import debug  # noqa
 from charmhelpers.fetch.python import packages  # noqa

charmhelpers/core/hookenv.py

@@ -17,8 +17,6 @@
 #
 # Authors:
 #  Charm Helpers Developers <juju@lists.ubuntu.com>
-
-from __future__ import print_function
 import copy
 from distutils.version import LooseVersion
 from enum import Enum

charmhelpers/fetch/python/debug.py

@@ -15,8 +15,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import print_function
-
 import atexit
 import sys