[#4841] Change statement to support psycopg2 2.8.x

Some users might find libssl issues (see #4837) for which an upgrade in
psycopg2 is required. While we are not porting the requirement upgrade
to 2.8, we make this change which is backwards compatible with psycopg2
2.7.3.2 (CKAN 2.8 pinned version) in case someone needs to upgrade the
requirement in their environment.

Just in case we fall back to the old syntax if there is an
AttributeError.

ckanext/datastore/backend/postgres.py

@@ -284,7 +284,10 @@ def _cache_types(context):
             # redo cache types with json now available.
             return _cache_types(context)
 
-        register_composite('nested', connection.connection, True)
+        try:
+            register_composite('nested', connection.connection.connection, True)
+        except AttributeError:
+            register_composite('nested', connection.connection, True)
 
 
 def _pg_version_is_at_least(connection, version):