"""create books table

Revision ID: 1234567890ab
Revises: 
Create Date: 2023-05-25 10:00:00.000000

"""
from alembic import op
import sqlalchemy as sa
from sqlalchemy.sql import func


# revision identifiers, used by Alembic
revision = '1234567890ab'
down_revision = None
branch_labels = None
depends_on = None


def upgrade():
    # Create books table
    op.create_table(
        'books',
        sa.Column('id', sa.Integer, primary_key=True, index=True),
        sa.Column('title', sa.String, index=True),
        sa.Column('author', sa.String, index=True),
        sa.Column('description', sa.String),
        sa.Column('published_year', sa.Integer),
        sa.Column('isbn', sa.String, unique=True),
        sa.Column('created_at', sa.DateTime, server_default=func.now()),
        sa.Column('updated_at', sa.DateTime, server_default=func.now(), onupdate=func.now())
    )


def downgrade():
    # Drop books table
    op.drop_table('books')
```

This Alembic migration script creates a new table called 'books' with the specified columns and constraints. The `upgrade()` function creates the table, while the `downgrade()` function drops the table. Note the use of `server_default=func.now()` for the `created_at` and `updated_at` columns to automatically set the timestamp on record creation and update, respectively.