Fix sqlalchemy 0.8 issues

Use import of exc instead exception.
Mutability tracking is changed in SQLAlchemy 0.8,
but it seems that sqlalchmy.types.MutableType is not needed
in classes Json and Failure from models.

Closes-bug: 1206669

Change-Id: Ib13c2575ead495b22ed5624e68e044e2f4927201

taskflow/persistence/backends/impl_sqlalchemy.py

@@ -27,7 +27,7 @@ import logging
 import time
 
 import sqlalchemy as sa
-from sqlalchemy import exceptions as sa_exc
+from sqlalchemy import exc as sa_exc
 from sqlalchemy import orm as sa_orm
 from sqlalchemy import pool as sa_pool
 

taskflow/persistence/backends/sqlalchemy/models.py

@@ -39,7 +39,7 @@ class TimestampMixin(object):
     updated_at = Column(DateTime, onupdate=timeutils.utcnow)
 
 
-class Json(types.TypeDecorator, types.MutableType):
+class Json(types.TypeDecorator):
     impl = types.Text
 
     def process_bind_param(self, value, dialect):
@@ -49,7 +49,7 @@ class Json(types.TypeDecorator, types.MutableType):
         return jsonutils.loads(value)
 
 
-class Failure(types.TypeDecorator, types.MutableType):
+class Failure(types.TypeDecorator):
     """Put misc.Failure object into database column.
 
     We convert Failure object to dict, serialize that dict into