title: psycopg2: insert multiple rows with one query
body: <p>I need to insert multiple rows with one query (number of rows is not constant), so I need to execute query like this one:</p>
  
  <pre><code>INSERT INTO t (a, b) VALUES (1, 2), (3, 4), (5, 6);
  </code></pre>
  
  <p>The only way I know is</p>
  
  <pre><code>args = [(1,2), (3,4), (5,6)]
  args_str = ','.join(cursor.mogrify("%s", (x, )) for x in args)
  cursor.execute("INSERT INTO t (a, b) VALUES "+args_str)
  </code></pre>
  
  <p>but I want some simpler way.</p>