title: Alembic: IntegrityError: "column contains null values" when adding non-nullable column
body: <p>I'm adding a column to an existing table. This new column is <code>nullable=False</code>.</p>
  
  <pre><code>op.add_column('mytable', sa.Column('mycolumn', sa.String(), nullable=False))
  </code></pre>
  
  <p>When I run the migration, it complains:</p>
  
  <pre><code>sqlalchemy.exc.IntegrityError: column "mycolumn" contains null values
  </code></pre>