title: Python 2 -> 3 Django migration causes field parameter type change
body: <p>We are transitioning a Django project from Django 1.8 -> 2.1 and Python 2.7 -> 3.6.</p>
  
  <p>In the old project version, there are Django models that looked like this, for example:</p>
  
  <pre><code># models.py
  
  from django.db import models
  
  class RowStatusModel(models.Model):
      active = models.BooleanField(default=True, db_column='is_active')
      # ...
      class Meta:
          abstract = True
  </code></pre>
  
  <p>Notice that <code>from __future__ import unicode_literals</code> is <em>not</em> used in this module.  That means that <code>db_column</code> is a Python 2 <code>str</code>, corresponding to <code>bytes</code> in Python 3.  The initial migration, 0001_initial.py, looks like this:</p>
  
  <pre><code># 0001_initial.py
  
  operations = [
      # ...
      ('row_ef', models.BooleanField(default=True, db_column=b'is_active')
      # ...
  ]
  </code></pre>
  
  <p>Notice the byte-literal <code>b'is_active</code>, which I suppose was done by Django in the interest of being more explicit, but am not sure.</p>
  
  <p>Now after transitioning most of the codebase with 2to3 and running <code>makemigrations</code>, Python 3 treats a string literal as what would be the unicode type in Python 2, and consequently generates a migration where the <code>db_column</code> is a string literal, for every single model that inherits from <code>RowStatusModel</code>:</p>
  
  <pre><code># migrations/0023_auto_20180827_1955.py 
  migrations.AlterField(
      # ...
      field=models.BooleanField(default=True, db_column='is_active')
  ), # ...
  </code></pre>
  
  <p><strong>What effect, if any</strong>, will this have on the database side when <code>./manage.py migrate</code> is ran?  Is the "change" purely on the Python side, or what side effects may be created?</p>
  
  <hr>
  
  <p>Database engine is <code>django.db.backends.postgresql</code>.</p>
  
  <hr>
  
  <p>I'm aware that we can just clone the RDS instance and revert back to that if <code>migrate</code> causes immediate problems, but I am more concerned about more subtle issues being introduced that would not be caught until much later on.</p>