fetched when joining/joinedloading across a many-to-many
relationship to a single-table-inheriting
subclass with a specific discriminator value, due to "secondary"
rows that would come back.  The "secondary" and right-side
tables are now inner joined inside of parenthesis for all
ORM joins on many-to-many relationships so that the left->right
join can accurately filtered.
[ticket:2369]

a CAST call that was added in 0.8.1 to help with driver issues,
which apparently is not compatible on 2000.
The CAST remains in place for SQL Server 2005 and greater.
[ticket:2747]

:class:`.ForeignKeyConstraint` will not render the ``DEFERRABLE`` keyword
on the MySQL dialect.  For a long time we left this in place because
a non-deferrable foreign key would act very differently than a deferrable
one, but some environments just disable FKs on MySQL, so we'll be less
opinionated here. [ticket:2721]

when ``clear()`` was called.
[ticket:2730]

foreign key paths to a table "B", to that table "B", would fail
to produce the "ambiguous join condition" error that would be
reported if you join table "A" directly to "B"; it would instead
produce a join condition with multiple criteria.
[ticket:2738]

schema as well as a local schema could produce wrong results
in the case where both schemas had a table of the same name.
[ticket:2728]

with legitimate iterable detection, [ticket:2726]

- inline the label check

- restore the rollback cleanup handler, pg8000 is mostly obsolete
as a dialect and the firebird drivers need it