Use datetime.max in no expirable entries

Author: WaVEV

django_mongodb_backend/cache.py

@@ -186,7 +186,7 @@ def incr(self, key, delta=1, version=None):
 
     def _get_expiration_time(self, timeout=None):
         if timeout is None:
-            return None
+            return datetime.max
         timestamp = self.get_backend_timeout(timeout)
         return datetime.fromtimestamp(timestamp, tz=timezone.utc)