text: Title: render_item method in env.py is not being called
  Body: There's an issue with the `sqlalchemy_utils` package. Special field types like UUID, Password from this library are not being recognized by alembic. There was a workaround in earlier versions of `flask-migrate` and that is to write custom `render_item` method in `env.py`:
  
  ```python
  def render_item(type_, obj, autogen_context):
      """Apply custom rendering for selected items."""
  
      if type_ == 'type' and isinstance(obj, sqlalchemy_utils.types.uuid.UUIDType):
          # add import for this type
          autogen_context.imports.add("import sqlalchemy_utils")
          autogen_context.imports.add("import uuid")
          return "sqlalchemy_utils.types.uuid.UUIDType(), default=uuid.uuid4"
  
      # default rendering for other objects
      return False
  ```
  
  This was resulting in versions being correct, e.g. sqlalchemy_utils imported and `Column type` is correct. 
  
  I upgraded to the latest `flask-migrate` from `2.3.1` and this workaround no longer works. Seems like the `render_item` method is not called at all. 
  
  