change datetime expression for 0 epoch seconds

Author: d-w-moore

irods/data_object.py

@@ -1,5 +1,5 @@
 import ast
-import datetime
+from datetime import datetime, UTC
 import enum
 import io
 import logging
@@ -64,7 +64,7 @@ class _repl_status(enum.Enum):
 
 # An appropriate reference datetime value for gauging replica age as part of
 # the default sort key in PRC4 and onward.
-_REFERENCE_DATETIME = datetime.datetime(1970, 1, 1, 0, 0, 0, tzinfo=datetime.timezone.utc)
+_REFERENCE_DATETIME = datetime.fromtimestamp(0, UTC)
 
 def _REPLICA_NUMBER_SORT_KEY_FN(row):
     return row[DataObject.replica_number]