Add Djanog 2.0 deprecated methods

FileField methods get_directory_name() and get_filename() was deprecated in Django 2.0, so adding them manually to keep support of previous Django versions.

private_storage/fields.py

@@ -12,6 +12,9 @@ from django.template.defaultfilters import filesizeformat
 from django.utils.translation import ugettext_lazy as _
 from .storage import private_storage
 
+import datetime
+from django.utils.encoding import force_str, force_text
+
 logger = logging.getLogger(__name__)
 
 
@@ -71,3 +74,15 @@ class PrivateFileField(models.FileField):
             subdirs = [self.get_directory_name()]
         dirs = list(subdirs) + [self.get_filename(filename)]
         return os.path.normpath(os.path.join(*dirs))
+
+    def get_directory_name(self):
+        '''
+        Added for compatibility with Django 2.0 where this method was removed
+        '''
+        return os.path.normpath(force_text(datetime.datetime.now().strftime(force_str(self.upload_to))))
+
+    def get_filename(self, filename):
+        '''
+        Added for compatibility with Django 2.0 where this method was removed
+        '''
+        return os.path.normpath(self.storage.get_valid_name(os.path.basename(filename)))