Add --diff flag in isort in order to display changes (#4469)

tox.ini

@@ -87,4 +87,4 @@ commands =
     pydocstyle {toxinidir}/celery
 
 [testenv:isort-check]
-commands = isort --project celery --order-by-type -rc -c {toxinidir}/celery {toxinidir}/t
+commands = isort --project celery --diff --order-by-type -rc -c {toxinidir}/celery {toxinidir}/t