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: Idb4a361d4d65ff53ecf33b8a2a6aa0d6f6ae1979

watcher/db/sqlalchemy/models.py

@@ -21,7 +21,6 @@ from oslo_serialization import jsonutils
 from sqlalchemy import Boolean
 from sqlalchemy import Column
 from sqlalchemy import DateTime
-from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy import Float
 from sqlalchemy import ForeignKey
 from sqlalchemy import Integer
@@ -94,7 +93,7 @@ class WatcherBase(models.SoftDeleteMixin,
         return d
 
 
-Base = declarative_base(cls=WatcherBase)
+Base = orm.declarative_base(cls=WatcherBase)
 
 
 class Goal(Base):