title: Timestamp.week vs. datetime.isocalendar
body: <p>I wonder why</p>
  <pre class="lang-py prettyprint-override"><code>&gt;&gt;&gt; pandas.Timestamp(dt.datetime(2009,1,6)).week
  </code></pre>
  <p>and</p>
  <pre class="lang-py prettyprint-override"><code>&gt;&gt;&gt; datetime.datetime(2009,1,6).isocalendar()[1]
  </code></pre>
  <p>don't give the same result.</p>