code: def upgrade():
      """Change default ``pool_slots`` to ``1`` and make pool_slots not nullable"""
      op.execute(
          "UPDATE task_instance SET pool_slots = 1 WHERE pool_slots IS NULL"
      )
      with op.batch_alter_table(
          "task_instance", schema=None
      ) as batch_op:
          batch_op.alter_column(
              "pool_slots",
              existing_type=sa.Integer,
              nullable=False,
              server_default="1",
          )
  
imports: from __future__ import annotations
  import sqlalchemy
  from alembic import op
label_desc: Use image node_id 1 for calling a global function with example usage: upgrade() without return types