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]

you're dealing with aliased() or with_polymorphic() you need to say "flat=True".  Just the one
flag though, "flat" implies "aliased".

- two suite of SQL assertions converted

might even be most of the tests we need (though dedicated sql tests would be needed anyway)

- to account for query._attributes/context.attributes, just pass
the attributes dict directly to the PathRegistry methods

made the check for recursion overflow in self-referential
eager joining too loose, missing a particular circumstance
where a subclass had lazy="joined" or "subquery" configured
and the load was a "with_polymorphic" against the base.
[ticket:2481]

- went through examples/ and cleaned out excess list() calls