Previously, it was impossible to have a Table that has
None for a schema name when the "schema" parameter on
MetaData was set.  A new symbol sqlalchemy.schema.BLANK_SCHEMA
is added which indicates that the schema name should unconditionally
be set to None.  In particular, this value must be passed within
cross-schema foreign key reflection, so that a Table which
is in the "default" schema can be represented properly.

Fixes: #3716
Change-Id: I3d24f99c22cded206c5379fd32a225e74edb7a8e

sort_tables_and_constraints function.
- The DDL generation system of :meth:`.MetaData.create_all`
and :meth:`.Metadata.drop_all` has been enhanced to in most
cases automatically handle the case of mutually dependent
foreign key constraints; the need for the
:paramref:`.ForeignKeyConstraint.use_alter` flag is greatly
reduced.  The system also works for constraints which aren't given
a name up front; only in the case of DROP is a name required for
at least one of the constraints involved in the cycle.
fixes #3282

to get all flake8 passing

Found using: https://github.com/intgr/topy

mark a constraint name as already having had a naming convention applied.
This token will be used by Alembic migrations as of Alembic 0.6.4
in order to render constraints in migration scripts with names marked
as already having been subject to a naming convention.
re: #2991

sqlalchemy-migrate uses SchemaVisitor. It was one of the only
objects that changed namespaces without compatibility that we use.

Would love to get this added to remove a work around we need at
https://review.openstack.org/#/c/66156/