- Fixed regression due to :ticket:`3282` where the ``tables`` collection

- Fixed regression due to :ticket:`3282` where the ``tables`` collection
passed as a keyword argument to the :meth:`.DDLEvents.before_create`,
:meth:`.DDLEvents.after_create`, :meth:`.DDLEvents.before_drop`, and
:meth:`.DDLEvents.after_drop` events would no longer be a list
of tables, but instead a list of tuples which contained a second
entry with foreign keys to be added or dropped.  As the ``tables``
collection, while documented as not necessarily stable, has come
to be relied upon, this change is considered a regression.
Additionally, in some cases for "drop", this collection would
be an iterator that would cause the operation to fail if
prematurely iterated.   The collection is now a list of table
objects in all cases and test coverage for the format of this
collection is now added.
fixes #3391