[postgres] adding support to serialized timedelta (#3444)

superset/utils.py

@@ -20,7 +20,7 @@ import sys
 import zlib
 
 from builtins import object
-from datetime import date, datetime, time
+from datetime import date, datetime, time, timedelta
 
 import celery
 from dateutil.parser import parse
@@ -278,6 +278,8 @@ def base_json_conv(obj):
         return float(obj)
     elif isinstance(obj, uuid.UUID):
         return str(obj)
+    elif isinstance(obj, timedelta):
+        return str(obj)
 
 
 def json_iso_dttm_ser(obj):