title: Alembic: Include constraint to reject strings with character
body: <p>I have a table that looks like this</p>
  
  <pre><code>&gt; select * from mytable
    id        value
  0  1  hello world
  1  2  hello_world
  2  3  hello+world
  </code></pre>
  
  <p>I'm trying to impose an alembic check constraint, where values in <code>value</code> cannot have the <code>:</code> character. How would I do this using an <code>alembic.op</code> object? What are the <code>upgrade()</code> and <code>downgrade()</code> functions?</p>
  
  <p>Edit: The DB I'm using is Mysql.</p>