Fix assertion failure when retrying to migrate a workflow history

When a first migration was reverted for any reason (e.g. ConflictError),
the assertion failed because __setstate__ was not called yet to fix the
class back to the old one.
10 jobs for arnau in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Master

03:19:08

passed ERP5.PerformanceTest-Master

00:31:31

failed ERP5.UnitTest-Master

02:59:41

failed ERP5.UnitTest-Master.Medusa

07:23:33

passed SlapOS.Eggs.UnitTest-Master.Python2

00:44:56

passed SlapOS.Eggs.UnitTest-Master.Python3

00:19:06

failed ERP5.UnitTest-Master

02:28:23

failed ERP5.UnitTest-Master

07:42:06

passed SlapOS.Eggs.UnitTest-Master.Python2

00:24:09

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:21