use simpler isoparse from dateutil to parse ISO8601 timestamps

avoids more complex behavior of general parser, which we don't need
since we are already specifying the general format spec

jupyter_client/jsonutil.py

@@ -11,7 +11,7 @@
 from datetime import datetime
 from typing import Any, Optional, Union
 
-from dateutil.parser import parse as _dateutil_parse
+from dateutil.parser import isoparse as _dateutil_parse
 from dateutil.tz import tzlocal
 
 next_attr_name = "__next__"  # Not sure what downstream library uses this, but left it to be safe