code: def alter_column_type(self, relation, column_name, new_column_type):
          
          kwargs = {
              'relation': relation,
              'column_name': column_name,
              'new_column_type': new_column_type,
          }
          self.execute_macro(
              ALTER_COLUMN_TYPE_MACRO_NAME,
              kwargs=kwargs
          )
docs: 1. Create a new column (w/ temp name and correct type)
          2. Copy data over to it
          3. Drop the existing column (cascade!)
          4. Rename the new column to existing column