Use str(Retry) to properly display retry time (related to #3330)

celery/app/trace.py

@@ -30,7 +30,7 @@ from celery._state import _task_stack
 from celery.app import set_default_app
 from celery.app.task import Task as BaseTask, Context
 from celery.exceptions import Ignore, Reject, Retry, InvalidTaskError
-from celery.five import monotonic
+from celery.five import monotonic, text_t
 from celery.utils.log import get_logger
 from celery.utils.nodenames import gethostname
 from celery.utils.objects import mro_lookup
@@ -174,8 +174,9 @@ class TraceInfo(object):
             signals.task_retry.send(sender=task, request=req,
                                     reason=reason, einfo=einfo)
             info(LOG_RETRY, {
-                'id': req.id, 'name': task.name,
-                'exc': safe_repr(reason.exc),
+                'id': req.id,
+                'name': task.name,
+                'exc': text_t(reason),
             })
             return einfo
         finally: