for #3175.  fixes #3175 (for now)

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

:class:`.Query` and in other situations where selects or joins
were aliased (such as joined table inheritance) could fail if a
user-defined :class:`.Column` subclass were used in the expression.
In this case, the subclass would fail to propagate ORM-specific
"annotations" along needed by the adaptation.  The "expression
annotations" system has been corrected to account for this case.
[ticket:2918]

the import structure of many core modules.
``sqlalchemy.schema`` and ``sqlalchemy.types``
remain in the top-level package, but are now just lists of names
that pull from within ``sqlalchemy.sql``.  Their implementations
are now broken out among ``sqlalchemy.sql.type_api``, ``sqlalchemy.sql.sqltypes``,
``sqlalchemy.sql.schema`` and ``sqlalchemy.sql.ddl``, the last of which was
moved from ``sqlalchemy.engine``.  ``sqlalchemy.sql.expression`` is also
a namespace now which pulls implementations mostly from ``sqlalchemy.sql.elements``,
``sqlalchemy.sql.selectable``, and ``sqlalchemy.sql.dml``.
Most of the "factory" functions
used to create SQL expression objects have been moved to classmethods
or constructors, which are exposed in ``sqlalchemy.sql.expression``
using a programmatic system.  Care has been taken such that all the
original import namespaces remain intact and there should be no impact
on any existing applications.   The rationale here was to break out these
very large modules into smaller ones, provide more manageable lists
of function names, to greatly reduce "import cycles" and clarify the
up-front importing of names, and to remove the need for redundant
functions and documentation throughout the expression package.