Don't use private API to get query criteria

Another change for SQLAlchemy 1.4 is the mechanics
of how the ``QueryContext`` object works.   Update the
update_match extension such that it does not rely upon the
``QueryContext`` object and uses public APIs instead.

Change-Id: I27e20e59bce8c98b5a507c617364652aa6eb33f5

oslo_db/sqlalchemy/update_match.py

@@ -484,11 +484,10 @@ def _update_stmt_from_query(mapper, query, values):
             mapper.column_attrs[key], value
         ) for key, value in values.items()
     )
-    query = query.enable_eagerloads(False)
-    context = query._compile_context()
-    primary_table = context.statement.froms[0]
+    primary_table = inspect(query.column_descriptions[0]['entity']).local_table
+    where_criteria = query.whereclause
     update_stmt = sql.update(primary_table,
-                             context.whereclause,
+                             where_criteria,
                              upd_values)
     return update_stmt