Dont import contenttype to avoid import issues when contenttypes is not in installed apps

[miki725]

that should fix:

Traceback (most recent call last):                                  
  File "/home/serkan/.local/share/virtualenvs/slides-2sl0-4FG/src/sliderepl/sliderepl/core.py", line 292, in run                        
    exec_(co, environ)            
  File "<input>", line 1, in <module>                               
  File "/home/serkan/.local/share/virtualenvs/slides-2sl0-4FG/lib/python3.6/site-packages/url_filter/filtersets/__init__.py", line 5, in <module>
    from .django import *  # noqa 
  File "/home/serkan/.local/share/virtualenvs/slides-2sl0-4FG/lib/python3.6/site-packages/url_filter/filtersets/django.py", line 6, in <module>
    from django.contrib.contenttypes.fields import GenericForeignKey                                                                    
  File "/home/serkan/.local/share/virtualenvs/slides-2sl0-4FG/lib/python3.6/site-packages/django/contrib/contenttypes/fields.py", line 5, in <module>
    from django.contrib.contenttypes.models import ContentType      
  File "/home/serkan/.local/share/virtualenvs/slides-2sl0-4FG/lib/python3.6/site-packages/django/contrib/contenttypes/models.py", line 139, in <module>
    class ContentType(models.Model):                                
  File "/home/serkan/.local/share/virtualenvs/slides-2sl0-4FG/lib/python3.6/site-packages/django/db/models/base.py", line 118, in __new__
    "INSTALLED_APPS." % (module, name)                              
RuntimeError: Model class django.contrib.contenttypes.models.ContentType doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.