Fixed bug where in Py2K a unicode literal would not be accepted as the
string name of a class or other argument within declarative using
:func:`.backref` on :func:`.relationship`.

amends commit e6f67f48054d906856f879bc1803ea639aa4b670

setups, where the attempt to check for "duplicate class name" in
declarative would hit upon a not-totally-cleaned-up weak reference
related to some other class being removed; the check here now ensures
the weakref still references an object before calling upon it further.
fixes #3208

to get all flake8 passing

target of :paramref:`.relationship.secondary` for the purposes
of creating very complex :func:`.relationship` join conditions.
The change includes adjustments to query joining, joined eager loading
to not render a SELECT subquery, changes to lazy loading such that
the "secondary" target is properly included in the SELECT, and
changes to declarative to better support specification of a
join() object with classes as targets.

automatic reflection support for the "secondary" table referred
to by a :func:`.relationship`.   "secondary", when specified
either as a string table name, or as a :class:`.Table` object with
only a name and :class:`.MetaData` object will also be included
in the reflection process when :meth:`.DeferredReflection.prepare`
is called. [ticket:2865]
- clsregistry._resolver() now uses a stateful _class_resolver()
class in order to handle the work of mapping strings to
objects.   This is to provide for simpler extensibility, namely
a ._resolvers collection of ad-hoc name resolution functions;
the DeferredReflection class adds its own resolver here in order
to handle relationship(secondary) names which generate new
Table objects.