[sqlalchemy-20] Fix import of "ASSOCIATION_PROXY"

The "ASSOCIATION_PROXY" symbol is now a string member of class
``AssociationProxyExtensionType`` in SQLAlchemy 2.0

Change-Id: I06ae8d3f0a28d5f8575cb56cad2c64e3a2a291db
Closes-Bug: #2004184

neutron_lib/db/utils.py

@@ -15,7 +15,7 @@ import functools
 from oslo_db import exception as db_exc
 from oslo_utils import excutils
 import sqlalchemy
-from sqlalchemy.ext.associationproxy import ASSOCIATION_PROXY
+from sqlalchemy.ext import associationproxy
 from sqlalchemy.orm import exc
 from sqlalchemy.orm import properties
 
@@ -152,8 +152,14 @@ def filter_non_model_columns(data, model):
     """
     mapper = sqlalchemy.inspect(model)
     columns = set(c.name for c in mapper.columns)
+    try:
+        _association_proxy = associationproxy.ASSOCIATION_PROXY
+    except AttributeError:
+        # SQLAlchemy 2.0
+        _association_proxy = (
+            associationproxy.AssociationProxyExtensionType.ASSOCIATION_PROXY)
     columns.update(d.value_attr for d in mapper.all_orm_descriptors
-                   if d.extension_type is ASSOCIATION_PROXY)
+                   if d.extension_type is _association_proxy)
     return dict((k, v) for (k, v)
                 in data.items() if k in columns)