expose SchemaVisitor in the compatibility layer

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/

diff --git a/lib/sqlalchemy/schema.py b/lib/sqlalchemy/schema.py
index 1721415396db2b1bd24f20c44a18b9da1fb5ed27..9e647e595292233464c020840c480858c3a42aaf 100644 (file)
--- a/lib/sqlalchemy/schema.py
+++ b/lib/sqlalchemy/schema.py
@@ -8,6 +8,10 @@
 
 """
 
+from .sql.base import (
+    SchemaVisitor
+    )
+
 
 from .sql.schema import (
     CheckConstraint,