Test Celery's latest RC (#3938)

Author: sentrivana

tox.ini

@@ -391,7 +391,7 @@ deps =
     celery-v5.3: Celery~=5.3.0
     celery-v5.4: Celery~=5.4.0
     # TODO: update when stable is out
-    celery-v5.5: Celery==5.5.0rc3
+    celery-v5.5: Celery==5.5.0rc4
     celery-latest: Celery
 
     celery: newrelic