[1.7.x] Fixed #22350 -- Consistently serialize bytes and text in migrations.

Thanks to @treyhunner and Loïc for their suggestions and review.

Backport of 72d3889db4 from master

tests/migrations/test_migrations/0001_initial.py

@@ -1,3 +1,6 @@
+# encoding: utf8
+from __future__ import unicode_literals
+
 from django.db import migrations, models