aggregations: turn agg query results into dictionaries

* because we're reusing parts of the aggregation query results directly,
  and per default they're wrapped in DSL objects (e.g. AttrList) that
  aren't JSON-serializable
* see: elastic/elasticsearch-dsl-py#913

invenio_stats/aggregations.py

@@ -341,7 +341,8 @@ def agg_iter(self, dt):
                 ignore_cache=True,
             )
             for aggregation in results.aggregations["terms"].buckets:
-                doc = aggregation.top_hit.hits.hits[0]["_source"]
+                doc = aggregation.top_hit.hits.hits[0]["_source"].to_dict()
+                aggregation = aggregation.to_dict()
                 interval_date = datetime.strptime(
                     doc["timestamp"], "%Y-%m-%dT%H:%M:%S"
                 ).replace(**dict.fromkeys(INTERVAL_ROUNDING[self.interval], 0))