Add a stub implementation of StateFilter.approx_difference

synapse/storage/state.py

@@ -193,6 +193,14 @@ class StateFilter:
                 include_others=True,
             )
 
+    def approx_difference(self, subtrahend: "StateFilter") -> "StateFilter":
+        """
+        Stub implementation! Satisfies the condition that it's an overestimate.
+        """
+        if self == subtrahend:
+            return StateFilter.none()
+        return self
+
     def make_sql_filter_clause(self) -> Tuple[str, List[str]]:
         """Converts the filter to an SQL clause.