title: How to use alter_column in alembic?
body: <p>I am writing a migration in alembic but seems impossible to me the change the value of server_defaults from something to nothing.</p>
  
  <p>My code:</p>
  
  <pre><code>op.alter_column("foo", sa.Column("bar", sa.DateTime(timezone=False), server_default=None, nullable=True))
  </code></pre>
  
  <p>If i check after the migration the default is still NOW()</p>