[sqlalchemy-20] Use sqlalchemy.orm.DeclarativeBase

``declarative_base()`` is superseded by ``DeclarativeBase`` class.
More information in [1].

[1]https://docs.sqlalchemy.org/en/20/changelog/whatsnew_20.html#step-one-orm-declarative-base-is-superseded-by-orm-declarativebase

Closes-Bug: #2051171
Change-Id: I5ddf9573572e52fe87bbfde9732c5af9ce398188

neutron_lib/db/model_base.py

@@ -110,4 +110,10 @@ class NeutronBaseV2(_NeutronBase):
         return cls.__name__.lower() + 's'
 
 
-BASEV2 = declarative.declarative_base(cls=NeutronBaseV2)
+try:
+    # SQLAlchemy 2.0
+    class BASEV2(orm.DeclarativeBase, NeutronBaseV2):
+        pass
+except AttributeError:
+    # SQLAlchemy < 2.0
+    BASEV2 = declarative.declarative_base(cls=NeutronBaseV2)

neutron_lib/tests/unit/db/test_standard_attr.py

@@ -15,6 +15,7 @@
 import gc
 
 from sqlalchemy.ext import declarative
+from sqlalchemy import orm
 import testtools
 
 from neutron_lib.db import standard_attr
@@ -29,8 +30,17 @@ class StandardAttrTestCase(base.BaseTestCase):
     def _make_decl_base(self):
         # construct a new base so we don't interfere with the main
         # base used in the sql test fixtures
-        return declarative.declarative_base(
-            cls=standard_attr.model_base.NeutronBaseV2)
+        try:
+            # SQLAlchemy 2.0
+            class BaseV2(orm.DeclarativeBase,
+                         standard_attr.model_base.NeutronBaseV2):
+                pass
+
+            return BaseV2
+        except AttributeError:
+            # SQLAlchemy < 2.0
+            return declarative.declarative_base(
+                cls=standard_attr.model_base.NeutronBaseV2)
 
     def test_standard_attr_resource_model_map(self):
         rs_map = standard_attr.get_standard_attr_resource_model_map()

neutron_lib/tests/unit/db/test_utils.py

@@ -26,14 +26,23 @@ from neutron_lib import exceptions as n_exc
 from neutron_lib.tests import _base as base
 
 
-class FakePort(declarative.declarative_base(cls=models.ModelBase)):
+try:
+    # SQLAlchemy 2.0
+    class ModelBaseV2(orm.DeclarativeBase, models.ModelBase):
+        pass
+except AttributeError:
+    # SQLAlchemy < 2.0
+    ModelBaseV2 = declarative.declarative_base(cls=models.ModelBase)
+
+
+class FakePort(ModelBaseV2):
     __tablename__ = 'fakeports'
     port_id = sa.Column(sa.String(36), primary_key=True)
     name = sa.Column(sa.String(64))
     status = sa.Column(sa.String(16), nullable=False)
 
 
-class FakeRouter(declarative.declarative_base(cls=models.ModelBase)):
+class FakeRouter(ModelBaseV2):
     __tablename__ = 'fakerouters'
     router_id = sa.Column(sa.String(36), primary_key=True)
     gw_port_id = sa.Column(sa.String(36), sa.ForeignKey(FakePort.port_id))