title: convert selected datetime to date in sqlalchemy
body: <p>I have a database of test records with one column 'test_time' defined as datetime. I want to query how many distinct dates there are as I want to dump test results to csv according to dates. I now have the following:</p>
  
  <pre><code>distinct_dates = list(session.query(Test_Table.test_time).distinct())
  </code></pre>
  
  <p>But this gives me a list of datetime not date. Certainly I can convert it in Python but when I am using sqlite. I did this <code>SELECT DISTINCT DATE(test_time) FROM Test_Table</code>. I cannot come up with the equivalent in sqlalchemy. </p>