sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

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

- rework the event system so that event modules load after their
targets, dependencies are reversed
- create an improved strategy lookup system for the ORM
- rework the ORM to have very few import cycles
- move out "importlater" to just util.dependency
- other tricks to cross-populate modules in as clear a way as possible

as possible

- went through examples/ and cleaned out excess list() calls

construct after it's been constructed already.  This is not
a pattern for normal use but we like to change the setting
for demonstration purposes in tutorials.