a column setup on the ABC base which had a different attribute
name vs. column name would not be correctly mapped on the final
base class.   The failure on 0.9 would be silent whereas on
1.0 it raised an ArgumentError, so may not have been noticed
prior to 1.0.
fixes #3480

event where its invocation was moved to be after the class manager's
instrumentation of the class, which is the opposite of what the
documentation for the event explicitly states.  The rationale for the
switch was due to Declarative taking the step of setting up
the full "instrumentation manager" for a class before it was mapped
for the purpose of the new ``@declared_attr`` features
described in :ref:`feature_3150`, but the change was also made
against the classical use of :func:`.mapper` for consistency.
However, SQLSoup relies upon the instrumentation event happening
before any instrumentation under classical mapping.
The behavior is reverted in the case of classical and declarative
mapping, the latter implemented by using a simple memoization
without using class manager.
fixes #3388

fixes #3383

and ``__declare_last__`` accessors where these would no longer be
called on the superclass of the declarative base.
fixes #3383

of a declarative inheritance hierarchy would prevent attributes
and configuration being correctly propagated from the base class
to the inheriting class.
fixes #3219 fixes #3240

subclass, the value returned by the descriptor is not available
because the superclass is already mapped with the InstrumentedAttribute,
until the subclass is mapped.  We add a setattr() to set up that
attribute so that the __mapper_args__ hook and possibly others
have access to the "cascaded" version of the attribute within
the call.

that are now affixed to _MapperConfig
- declarative now creates column copies ahead of time
so that they are ready to go for a declared_attr
- overhaul of declared_attr; memoization, cascading modifier
- A relationship set up with :class:`.declared_attr` on
a :class:`.AbstractConcreteBase` base class will now be configured
on the abstract base mapping automatically, in addition to being
set up on descendant concrete classes as usual.
fixes #2670
- The :class:`.declared_attr` construct has newly improved
behaviors and features in conjunction with declarative.  The
decorated function will now have access to the final column
copies present on the local mixin when invoked, and will also
be invoked exactly once for each mapped class, the returned result
being memoized.   A new modifier :attr:`.declared_attr.cascading`
is added as well. fixes #3150
- the original plan for #3150 has been scaled back; by copying
mixin columns up front and memoizing, we don't actually need
the "map properties later" thing.
- full docs + migration notes