If you are building a view on catalog tables (as opposed to repository tables), then you don't need the namespace part at all. Just supply the schema.table_name. If your table name is _ETRAG and its really in schema SYSTEM (which seems a bit odd) then the syntax would be:
query = "SELECT REFID, LIFNR FROM SYSTEM._ETRAG ";
If you just have one table, you don't need the table alias of T0. Of course if you just have one table, do you really need a view? Also you don't have to quote the field names, table names or schema if they are all upper case and don't contain any special characters. This is generally the case with replicated tables from ERP.