implicitly initialized to None via first access; this action,
which has always resulted in a population of the attribute,
now emits an attribute event just like any other attribute set
operation and generates the same kind of history as one.  Additionally,
many mapper internal operations will no longer implicitly generate
these "None" values when various never-set attributes are checked.
These are subtle behavioral fixes to attribute mechanics which provide
a better solution to the problem of :ticket:`3060`, which also
involves recognition of attributes explicitly set to ``None``
vs. attributes that were never set.
fixes #3061

Found using: https://github.com/intgr/topy

be used to manufacture objects that behave as though they were loaded
from a session, then detached.   Attributes that aren't present
are marked as expired, and the object can be added to a Session
where it will act like a persistent one. fix #3017

not be superseded.  both have a potential use.

now can accomodate additional information about the "reason" for
the exception; the :class:`.Session` now adds some detail to it
when the exception occurs within an autoflush.  This approach
is taken as opposed to combining :class:`.FlushError` with
a Python 3 style "chained exception" approach so as to maintain
compatibility both with Py2K code as well as code that already
catches ``IntegrityError`` or similar.