Django 1.7 migration files do not support Python 3.2

[Trey Hunner]

Steps to reproduce:

1. Run "python manage.py makemigrations appname" on a new app
2. Run "python3.2 manage.py migrate" (with Django installed in python3.2 site-packages)
3. View traceback due to syntax error. Example:

  File "/.../django-email-log/email_log/migrations/0001_initial.py", line 14
    (u'id', models.AutoField(verbose_name=u'ID', serialize=False, auto_created=True, primary_key=True)),
         ^
SyntaxError: invalid syntax

The syntax error is due to the use of the u prefix in front of string literals.

In my opinion, Python 3.2 support is unimportant because Python 3.3 made so many syntactic improvements. However, Python 3.2 support is currently noted in the documentation.

[loic84]

I'm not too fond of it, but here is a tentative patch, note the generated migrations are hideous.

POC ​https://github.com/loic/django/tree/ticket22350.

[Simon Charette]

Created a ​PR based on Loïc's initial patch relying on from __future__ import unicode_literals.

[Simon Charette <charette.s@…>]

In 72d3889db4eb3a14acb94f613edd79f0f27d26e3:

Fixed #22350 -- Consistently serialize bytes and text in migrations.

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

[Simon Charette <charette.s@…>]

In 9fb61cb64a18924b7f252677166533f747884b48:

[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