Native migrations created with Python 2 differ from ones created with
Python 3. More importantly, they failed when used in Python 3, but not
vice versa. This is caused by field names being marked as byte strings
when using Python 2.

This rewrites the native migrations, which probably isn't a bad thing
either, because they were created with a pre-RC2 version of Django, and
Django 1.7 has since seen a few more migration-related fixes.

It's likely Django now does the correct thing and generates Python 2 and
3 compatible migrations under both versions of Python.