Fixed regression caused by :ticket:`2677` whereby calling
:meth:`.Session.delete` on an object that was already flushed as
deleted in that session would fail to set up the object in the
identity map (or reject the object), causing flush errors as the
object were in a state not accommodated by the unit of work.
The pre-1.1 behavior in this case has been restored, which is that
the object is put back into the identity map so that the DELETE
statement will be attempted again, which emits a warning that the number
of expected rows was not matched (unless the row were restored outside
of the session).

Change-Id: I9a8871f82cb1ebe67a7ad54d888d5ee835a9a40a
Fixes: #3839

would still potentially cause persistence conflicts on the next
transaction, because the instance would not be checked that it
was expired.   This fix will resolve a large class of cases that
erronously cause the "New instance with identity X conflicts with
persistent instance Y" error.
fixes #3677

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing