unambiguous tracking of all object lifecycle state transitions
in terms of the :class:`.Session` itself, e.g. pending,
transient,  persistent, detached.   The state of the object
within each event is also defined.
fixes #2677
- Added a new session lifecycle state :term:`deleted`.  This new state
represents an object that has been deleted from the :term:`persistent`
state and will move to the :term:`detached` state once the transaction
is committed.  This resolves the long-standing issue that objects
which were deleted existed in a gray area between persistent and
detached.   The :attr:`.InstanceState.persistent` accessor will
**no longer** report on a deleted object as persistent; the
:attr:`.InstanceState.deleted` accessor will instead be True for
these objects, until they become detached.
- The :paramref:`.Session.weak_identity_map` parameter is deprecated.
See the new recipe at :ref:`session_referencing_behavior` for
an event-based approach to maintaining strong identity map behavior.
references #3517

where we refer to the "weak_identity_map" option, and add additional
exposition in the session documentation which refers to it.
fixes #3517

in callcounts specific to the "expiration" of objects, as in
the "auto expire" feature of :meth:`.Session.commit` and
for :meth:`.Session.expire_all`, as well as in the "cleanup" step
which occurs when object states are garbage collected.
fixes #3307

- don't do a bool on identity map since it calls __len__

now returns lists for ``items()`` and ``values()`` in Py3K.
Early porting to Py3K here had these returning iterators, when
they technically should be "iterable views"..for now, lists are OK.

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing