to get all flake8 passing

of work such that loading for related many-to-one objects is slightly
more aggressive, in the case of a graph of self-referential objects
that are to be deleted; the load of related objects is to help
determine the correct order for deletion if passive_deletes is
not set.
- revert the changes to test_delete_unloaded_m2o, these deletes do in fact
need to occur in the order of the two child objects first.

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

AttributeErrors or KeyErrors that should raise during mapper
configuration due to user errors.  The catch for attribute/keyerror
has been made more specific to not include the configuration step.
fixes #3047

to disable autoflush, in the case that the attribute needs to lazy-load
the "old" value, as in when replacing one-to-one values or some
kinds of many-to-one.  A flush at this point otherwise occurs
at the point that the attribute is None and can cause NULL violations.
[ticket:2921]

When the class_ passed is not a mapped class but is actually an iterable, the string formatting operation fails with a TypeError, and the expected ArgumentError is not raised. Calling code which is using reflection and expects this error will fail (e.g. the sadisplay module).