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: Ic1c5dfa969941b8d7a88dfc6586d1046d0fdd33f

unit_tests/test_lib_charm_openstack_manila_ganesha.py

@@ -12,9 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import absolute_import
-from __future__ import print_function
-
 import mock
 
 import charms_openstack.test_utils as test_utils

unit_tests/test_manila_ganesha_handlers.py

@@ -12,9 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from __future__ import absolute_import
-from __future__ import print_function
-
 # import json
 # import mock