for memoization on a class that uses slots.
- apply many more __slots__.  mem use for nova now at 46% savings

to get all flake8 passing

(e.g. ``x[:] = [...]``) would fail on Py3k.

caused a user-provided "getter" to no longer receive values of ``None``
when fetching scalar values from a target that is non-present.  The
check for None introduced by this change is now moved into the default
getter, so a user-provided getter will also again receive values of
None.
re: #2810

around for a long time
- association proxy now returns None for proxied scalar that is also None, rather
than raising AttributeError. [ticket:2810]

scalar values, for comparisons to None to also take into account
the association record itself being non-present, in addition to the
existing test for the scalar endpoint on the association record
being NULL.  Previously, comparing ``Cls.scalar == None`` would return
records for which ``Cls.associated`` were present and
``Cls.associated.scalar`` is None, but not rows for which
``Cls.associated`` is non-present.  More significantly, the
inverse operation ``Cls.scalar != None`` *would* return ``Cls``
rows for which ``Cls.associated`` was non-present.

Additionally, added a special use case where you
can call ``Cls.scalar.has()`` with no arguments,
when ``Cls.scalar`` is a column-based value - this returns whether or
not ``Cls.associated`` has any rows present, regardless of whether
or not ``Cls.associated.scalar`` is NULL or not.
[ticket:2751]