title: sqlalchemy update column with an expression
body: <p>I want to update a row setting the result of an expression on a column, ex:</p>
  
  <pre><code>MyTable.query.filter_by(id=the_id).update({
    "my_col": "(now() at time zone 'utc')"
  })
  </code></pre>
  
  <p>This code give me the following error : </p>
  
  <pre><code>(DataError) invalid input syntax for type timestamp: "(now() at time zone 'utc')"
  LINE 1: ...12bf98a-1ee9-4958-975d-ee99f87f1d4a', my_col='(now() at...
                                                               ^
   'UPDATE my_table SET my_col=%(redeemed_at)s WHERE id = %(id_1)s' {'my_col': "(now() at time zone 'utc')", 
  </code></pre>
  
  <p>this should translate in sql to : </p>
  
  <pre><code>update my_table set my_col = (now() at time zone 'utc') where id = ?;
  </code></pre>
  
  <p>this SQL statement works when run from the console</p>