postgresql: fix get_version() for older PostgreSQL versions

The bare 'RETURN' statement in get_version() is caught as a syntax error
by PostgreSQL 12.x. Return the integer literal using a simple SQL
statement instead.

kcidb/db/postgresql/v04_00.py

@@ -147,7 +147,7 @@ def set_schema_version(self, version):
                     RETURNS integer
                     LANGUAGE SQL
                     IMMUTABLE
-                    RETURN %s
+                    AS 'SELECT %s'
                 """), (number, ))
 
     def get_schema_version(self):