Cleaned up example migration files in docs

docs/ref/migration-operations.txt

@@ -280,7 +280,9 @@ example of using ``RunPython`` to create some initial objects on a ``Country``
 model::
 
     # -*- coding: utf-8 -*-
-    from django.db import models, migrations
+    from __future__ import unicode_literals
+
+    from django.db import migrations, models
 
     def forwards_func(apps, schema_editor):
         # We get the model from the versioned app registry;

docs/topics/migrations.txt

@@ -450,7 +450,9 @@ Then, open up the file; it should look something like this::
 
     # -*- coding: utf-8 -*-
     # Generated by Django A.B on YYYY-MM-DD HH:MM
-    from django.db import models, migrations
+    from __future__ import unicode_literals
+
+    from django.db import migrations, models
 
     class Migration(migrations.Migration):
         initial = True
@@ -478,7 +480,9 @@ and realized that not everyone has first and last names). All we
 need to do is use the historical model and iterate over the rows::
 
     # -*- coding: utf-8 -*-
-    from django.db import models, migrations
+    from __future__ import unicode_literals
+
+    from django.db import migrations, models
 
     def combine_names(apps, schema_editor):
         # We can't import the Person model directly as it may be a newer