title: Alembic: How to add unique constraint to existing column
body: <p>I have a table 'test' having a column 'Name' with no constraints. I need to <code>ALTER</code> this column by giving it a <code>UNIQUE</code> constraint. How should I do it?</p>
  
  <p>Should I use <code>op.alter_column('???')</code> or <code>create_unique_constraint('???')</code>?
  Isn't create_unique_constraint for new column and not for existing one?</p>