title: SqlAlchemy group_by and return max date
body: <p>I have a table such has</p>
  
  <pre><code>identifier date        value
  A          2017-01-01  2 
  A          2017-01-02  1
  A          2017-01-03  7
  B          2017-01-01  2 
  B          2017-01-02  7
  B          2017-01-03  3
  </code></pre>
  
  <p>I am trying to select the max date of each identifier such as  I get :</p>
  
  <pre><code>identifier date        value
  A          2017-01-03  7
  B          2017-01-03  3
  </code></pre>
  
  <p>thank you</p>