Resolve MovedIn20Warning

Adjust the import according to the following warning.

MovedIn20Warning: The ``declarative_base()`` function is now available
as sqlalchemy.orm.declarative_base(). (deprecated since: 2.0)
(Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)

Change-Id: Iecbb574937f48c6e36a57ccab37f7f351cd14419

cinder/db/sqlalchemy/models.py

@@ -26,11 +26,11 @@ from oslo_utils import timeutils
 import sqlalchemy as sa
 # imports needed for cinderlib
 from sqlalchemy import Column, String, Text  # noqa: F401
-from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy import func
 from sqlalchemy import schema
 from sqlalchemy.sql import expression
-from sqlalchemy.orm import backref, column_property, relationship, validates
+from sqlalchemy.orm import backref, column_property, declarative_base, \
+    relationship, validates
 
 
 CONF = cfg.CONF