Added a bunch of missing unicode_literals

Refs #22564.

django/db/migrations/operations/base.py

@@ -1,3 +1,6 @@
+from __future__ import unicode_literals
+
+
 class Operation(object):
     """
     Base class for migration operations.

django/db/migrations/operations/fields.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 from django.db import router
 from django.db.models.fields import NOT_PROVIDED
 from django.utils import six

django/db/migrations/operations/models.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 from django.db import models, router
 from django.db.models.options import normalize_together
 from django.db.migrations.state import ModelState

django/db/migrations/operations/special.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 from .base import Operation