fix string replacement support for sql

string replacement was broken by
https://github.com/farcepest/MySQLdb1/commit/87d1145c0d6ee4f5a8ecf6d5c62d2479b9cf27ea

This fix is to bring the string replacement support back:
  sql = "SELECT * FROM table_name WHERE column = %s"
  cursor.execute(sql, 'passed_variable')