title: Using translate field for 'unique' rises NameError, django-parler
body: <p>I've recently installed <code>django-parler==2.2</code> and when I rewrite my model to inherit from <code>TranslatableModel</code> and then try to <code>makemigrations</code> I get the following error:</p>
  <p><code>main_menu_slug = models.SlugField(max_length=120, unique=main_menu_item)</code>
  <code>NameError: name 'main_menu_item' is not defined</code></p>
  <p>Everything runs fine when I inherit from <code>models.Model</code>. Here is the code:</p>
  <pre><code>class MenuItem(TranslatableModel):
  
      translations = TranslatedFields (
          main_menu_item = models.CharField(max_length=60),
          main_menu_slug = models.SlugField(max_length=120, unique=main_menu_item)
      )
  </code></pre>
  <p>I've just started using this module, so possibly it is something trivial but I couldn't find the solution. What I am doing wrong?</p>
  <p>Full Traceback:</p>
  <pre><code>Traceback (most recent call last):
    File &quot;manage.py&quot;, line 22, in &lt;module&gt;
      main()
    File &quot;manage.py&quot;, line 18, in main
      execute_from_command_line(sys.argv)
    File &quot;C:\project\alfa\lib\site-packages\django\core\management\__init__.py&quot;, line 401, in execute_from_command_line
      utility.execute()
    File &quot;C:\project\alfa\lib\site-packages\django\core\management\__init__.py&quot;, line 377, in execute
      django.setup()
    File &quot;C:\project\alfa\lib\site-packages\django\__init__.py&quot;, line 24, in setup
      apps.populate(settings.INSTALLED_APPS)
    File &quot;C:\project\alfa\lib\site-packages\django\apps\registry.py&quot;, line 114, in populate
      app_config.import_models()
    File &quot;C:\project\alfa\lib\site-packages\django\apps\config.py&quot;, line 211, in import_models
      self.models_module = import_module(models_module_name)
    File &quot;C:\Users\user\AppData\Local\Programs\Python\Python37\lib\importlib\__init__.py&quot;, line 127, in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
    File &quot;&lt;frozen importlib._bootstrap&gt;&quot;, line 1006, in _gcd_import
    File &quot;&lt;frozen importlib._bootstrap&gt;&quot;, line 983, in _find_and_load
    File &quot;&lt;frozen importlib._bootstrap&gt;&quot;, line 967, in _find_and_load_unlocked
    File &quot;&lt;frozen importlib._bootstrap&gt;&quot;, line 677, in _load_unlocked
    File &quot;&lt;frozen importlib._bootstrap_external&gt;&quot;, line 728, in exec_module
    File &quot;&lt;frozen importlib._bootstrap&gt;&quot;, line 219, in _call_with_frames_removed
    File &quot;C:\project\alfa\project\menus\models.py&quot;, line 16, in &lt;module&gt;
      class MenuItem(TranslatableModel):
    File &quot;C:\project\alfa\project\menus\models.py&quot;, line 20, in MenuItem
      main_menu_slug = models.SlugField(max_length=120, unique=main_menu_item)
  NameError: name 'main_menu_item' is not defined
  </code></pre>
  <p>Working code <code>models.Model</code>:</p>
  <pre><code>class MenuItem(models.Model):
      main_menu_item = models.CharField(max_length=60)
      main_menu_slug = models.SlugField(max_length=120, unique=main_menu_item)
  </code></pre>