to get all flake8 passing

distinguished for when it was actually the value ``False``.
The ``__abstract__`` flag needs to acutally evaluate to a True
value at the level being tested.
fixes #3097

mixin or abstract class when accessed, so that modifications made
to this dictionary by declarative itself won't conflict with that
of other mappings.  The dictionary is modified regarding the
``version_id_col`` and ``polymorphic_on`` arguments, replacing the
column within with the one that is officially mapped to the local
class/table. fixes #3062

fully usable within declarative relationship configuration, as its
string classname would not be available in the registry of classnames
at mapper configuration time.   The class now explicitly adds itself
to the class regsitry, and additionally both :class:`.AbstractConcreteBase`
as well as :class:`.ConcreteBase` set themselves up *before* mappers
are configured within the :func:`.configure_mappers` setup, using
the new :meth:`.MapperEvents.before_configured` event. [ticket:2950]

- Added new :meth:`.MapperEvents.before_configured` event which allows
an event at the start of :func:`.configure_mappers`, as well
as ``__declare_first__()`` hook within declarative to complement
``__declare_last__()``.

- modified how after_configured is invoked; we just make a dispatch()
not actually connected to any mapper.  this makes it easier
to also invoke before_configured correctly.

- improved the ComparableEntity fixture to handle collections that are sets.

new automap extension would fail if classes
were pre-arranged in single or potentially joined inheritance patterns.
The repaired joined inheritance issue could also potentially apply when
using :class:`.DeferredReflection` as well.

:class:`.DeferredReflection`
to define classes pending for reflection, if some subset of those
classes were discarded before the :meth:`.DeferredReflection.prepare`
method were called to reflect and map the class, a strong reference
to the class would remain held within the declarative internals.
This internal collection of "classes to map" now uses weak
references against the classes themselves.

attribute on declarative to simply refer directly to an :class:`.InstrumentedAttribute`
on a superclass or on the class itself, and it
acts more or less like a synonym; in 0.9, this fails to set up enough
bookkeeping to keep up with the more liberalized backref logic
from :ticket:`2789`.  Even though this use case was never directly
considered, it is now detected by declarative at the "setattr()" level
as well as when setting up a subclass, and the mirrored/renamed attribute
is now set up as a :func:`.synonym` instead. [ticket:2900]

:class:`.Column` is mapped multiple times under different properties
(which typically should be a :func:`.synonym` instead) or if two
or more :class:`.Column` objects are given the same name, raising
a warning if this condition is detected. [ticket:2828]