Fail loudly if failing to override this method

Since this base class is opt-in, having an empty default
implementation (rather than an error) doesn't make a lot of sense.

Author: PeterJCLaw

django_lightweight_queue/backends/base.py

@@ -38,4 +38,4 @@ def deduplicate(
         *,
         progress_logger: ProgressLogger = NULL_PROGRESS_LOGGER
     ) -> Tuple[int, int]:
-        pass
+        raise NotImplementedError()