sqlalchemy: Use built-in declarative

sqlalchemy.ext.declarative was deprecated in sqlalchemy 1.4.0, due to
the built-in implementations[1].

[1] https://github.com/sqlalchemy/sqlalchemy/commit/450f5c0d6519a439f40

Change-Id: Iccbe77a00fa07f424c4a487a20b6f10c44f6ae83

placement/db/sqlalchemy/models.py

@@ -14,7 +14,6 @@
 from oslo_db.sqlalchemy import models
 from oslo_log import log as logging
 from sqlalchemy import Column
-from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy import Float
 from sqlalchemy import ForeignKey
 from sqlalchemy import Index
@@ -31,7 +30,7 @@ class _Base(models.ModelBase, models.TimestampMixin):
     pass
 
 
-BASE = declarative_base(cls=_Base)
+BASE = orm.declarative_base(cls=_Base)
 
 
 class ResourceClass(BASE):