title: django import-export not working with mptt
body: <p>I'm having and issue trying to get import-export to work for mptt models.
  This is my code:</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>from django.db import models
  from mptt.models import MPTTModel, TreeForeignKey
  
  class MyModel(MPTTModel):
      name = models.CharField(max_length=255)
      parent = TreeForeignKey('self', on_delete=models.CASCADE, null=True, blank=True, related_name='children')
      active = models.BooleanField(default=True,null=False,blank=False)
      sort_order = models.PositiveIntegerField(default=0,null=True,blank=True)
  
      class MPTTMeta:
          order_insertion_by = ['name']
  
      class Meta:
          ordering = ['sort_order']
  
      def __str__(self):
          return self.name
  </code></pre>
  
  <p><strong>admin.py</strong></p>
  
  <pre><code>from django.contrib import admin
  from mptt.admin import MPTTModelAdmin, DraggableMPTTAdmin
  from .models import MyModel
  from import_export import resources
  from import_export.admin import ImportExportModelAdmin, ImportExportMixin
  
  class MyModelResource(resources.ModelResource):
  
      class Meta:
          model = MyModel
          exclude = ('lft','rght','tree_id','level')
  
  @admin.register(MyModel)
  class MyModelAdmin(ImportExportMixin, DraggableMPTTAdmin):
      resource_class = MyModelResource
      list_display = ('tree_actions','indented_title','active','sort_order')
      list_editable = ('active','sort_order')
      list_display_links = ('indented_title',)
  </code></pre>
  
  <p>When I export it puts the name field in the parent column. When I import again it fails on the parent field:</p>
  
  <pre><code>"MyModel.parent" must be a "MyModel" instance.
  </code></pre>
  
  <p>Any help is greatly appreciated</p>