Django 2.0 support for permissions.py

Using user.is_authenticated() and user.is_anonymous() as a method is deprecated since Django 2.0

private_storage/permissions.py

@@ -4,14 +4,26 @@ Possible functions for the ``PRIVATE_STORAGE_AUTH_FUNCTION`` setting.
 
 
 def allow_authenticated(private_file):
-    return private_file.request.user.is_authenticated()
-
+    try:
+        return private_file.request.user.is_authenticated()
+    except AttributeError:
+        # Using user.is_authenticated() and user.is_anonymous() as a method is deprecated since Django 2.0
+        return private_file.request.user.is_authenticated
+    
 
 def allow_staff(private_file):
     request = private_file.request
-    return request.user.is_authenticated() and request.user.is_staff
+    try:
+        return request.user.is_authenticated() and request.user.is_staff
+    except AttributeError:
+        # Using user.is_authenticated() and user.is_anonymous() as a method is deprecated since Django 2.0
+        return request.user.is_authenticated and request.user.is_staff
 
 
 def allow_superuser(private_file):
     request = private_file.request
-    return request.user.is_authenticated() and request.user.is_superuser
+    try:
+        return request.user.is_authenticated() and request.user.is_superuser
+    except AttributeError:
+        # Using user.is_authenticated() and user.is_anonymous() as a method is deprecated since Django 2.0
+        return request.user.is_authenticated and request.user.is_superuser