Adding celery50 testing using tox.
Adding celery5.0 testing using tox.
Add a minor fix where original dict subclass replaced with a plain dict after sanity checking the keys due to celery5.0 in callback dict.
Adding celery5.0 testing using tox.
Add a minor fix where original dict subclass replaced with a plain dict after sanity checking the keys due to celery5.0 in callback dict.