Resolve Django 3.0 compatibility with new import locations

private_storage/fields.py

@@ -6,6 +6,7 @@ import logging
 import os
 import posixpath
 import warnings
+import sys
 
 import django
 from django.core.exceptions import ValidationError
@@ -13,14 +14,20 @@ from django.core.files.uploadedfile import UploadedFile
 from django.db import models
 from django.template.defaultfilters import filesizeformat
 from django.utils.encoding import force_str, force_text
-from django.utils.six import string_types
 from django.utils.translation import ugettext_lazy as _
 
+
 from .storage import private_storage
 
 logger = logging.getLogger(__name__)
 
 
+try:
+    string_types = basestring,  # Python 2
+except NameError:
+    string_types = str,  # Python 3
+
+
 class PrivateFileField(models.FileField):
     """
     Filefield with private storage, custom filename and size checks.

private_storage/servers.py

@@ -10,16 +10,17 @@ from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
 from django.http import FileResponse, HttpResponse, HttpResponseNotModified
 from django.utils.http import http_date
-from django.utils.lru_cache import lru_cache
 from django.utils.module_loading import import_string
 from django.views.static import serve, was_modified_since
 
 try:
     # python 3
     from urllib.parse import quote
+    from functools import lru_cache  # Python 3
 except ImportError:
     # python 2
     from urllib import quote
+    from django.utils.lru_cache import lru_cache  # Django <3
 
 
 @lru_cache()