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: Ie859ac36b1dcc4bc08e43eef3d6ed4399033f9f4

adjutant/actions/migrations/0001_initial.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import unicode_literals
 
 from django.db import models, migrations
 import django.utils.timezone

adjutant/actions/migrations/0002_action_auto_approve.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 

adjutant/actions/migrations/0003_auto_20190610_0205.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-10 02:05
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 

adjutant/actions/migrations/0004_auto_20190610_0209.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-10 02:09
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import django.db.models.deletion

adjutant/api/migrations/0001_initial.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import unicode_literals
 
 from django.db import models, migrations
 import jsonfield.fields

adjutant/api/migrations/0002_auto_20160815_2249.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 

adjutant/api/migrations/0003_task_approved_by.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import jsonfield.fields

adjutant/api/migrations/0004_auto_20160929_0317.py

@@ -1,5 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 

adjutant/api/migrations/0005_auto_20190610_0209.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-10 02:09
-from __future__ import unicode_literals
 
 from django.db import migrations
 

adjutant/api/migrations/0006_auto_20190610_0209.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-10 02:09
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import django.db.models.deletion

adjutant/api/migrations/0007_auto_20190610_0209.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-10 02:09
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import django.db.models.deletion

adjutant/api/migrations/0008_auto_20190610_0209.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-10 02:09
-from __future__ import unicode_literals
 
 from django.db import migrations
 

adjutant/tasks/migrations/0001_initial.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-10 02:09
-from __future__ import unicode_literals
 
 import adjutant.tasks.models
 from django.db import migrations, models

adjutant/tasks/migrations/0002_auto_20190619_0613.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.21 on 2019-06-19 06:13
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import jsonfield.fields