Fixed a backends test with psycopg2 2.7.

tests/backends/tests.py

@@ -314,6 +314,7 @@ class PostgreSQLTests(TestCase):
         """
         Regression test for #18130 and #24318.
         """
+        import psycopg2
         from psycopg2.extensions import (
             ISOLATION_LEVEL_READ_COMMITTED as read_committed,
             ISOLATION_LEVEL_SERIALIZABLE as serializable,
@@ -324,7 +325,8 @@ class PostgreSQLTests(TestCase):
         # PostgreSQL is configured with the default isolation level.
 
         # Check the level on the psycopg2 connection, not the Django wrapper.
-        self.assertEqual(connection.connection.isolation_level, read_committed)
+        default_level = read_committed if psycopg2.__version__ < '2.7' else None
+        self.assertEqual(connection.connection.isolation_level, default_level)
 
         new_connection = connection.copy()
         new_connection.settings_dict['OPTIONS']['isolation_level'] = serializable