title: How to run post upgrade code in alembic upgrade head
body: <p>I am modifying a model(adding a new column) and I have a corresponding migration file. My goal is to extend this migration by adding some data to the new column. </p>
  
  <p>I tried to modify the migration file by adding my own sql code to achieve my goal. Sample modified almebic file contains code like below:</p>
  
  <pre><code># below is the only line added by alembic
  op.add_column('sample_table', sa.Column('new_column', sa.UnicodeText(), nullable=True))
  # below lines are my own
  my_obj = MyClass()
  table_item = my_obj.get_table_item(some_parameter)
  ...
  ...
  </code></pre>
  
  <p>The issue is that my function "get_table_item" operates on the same table as is operated upon by alembic in "op.add_column" line. Since alembic locks the table during alembic upgrade, my function "get_table_item" gets stuck because of this lock.
  Is there any way to achieve my goal without modifying my function "get_table_item"?</p>