Change type annotation to celery/utils/iso8601.py (#8752)

* add type annotation

* change type annotation

celery/utils/iso8601.py

@@ -50,7 +50,7 @@
 )
 
 
-def parse_iso8601(datestring: str) -> str:
+def parse_iso8601(datestring: str) -> datetime:
     """Parse and convert ISO-8601 string to datetime."""
     warn("parse_iso8601", "v5.3", "v6", "datetime.datetime.fromisoformat or dateutil.parser.isoparse")
     m = ISO8601_REGEX.match(datestring)