input: Write a python function to serialize a course seat product to a dictionary for JSON serialization.
  assert serialize_seat_for_commerce_api(mock_seat_obj1) == {'name': 'verified', 'currency': 'USD', 'price': 100, 'sku': 'TESTSKU123', 'bulk_sku': None, 'expires': '2023-12-31T23:59:59'}
  assert serialize_seat_for_commerce_api(mock_seat_obj2) == {'name': 'audit', 'currency': 'USD', 'price': 0, 'sku': 'TESTSKU456', 'bulk_sku': 'BULKSKU456', 'expires': None}
  assert serialize_seat_for_commerce_api(mock_seat_obj3) == {'name': 'credit', 'currency': 'EUR', 'price': 150, 'sku': 'TESTSKU789', 'bulk_sku': 'BULKSKU789', 'expires': '2023-11-30T23:59:59'}
output: def serialize_seat_for_commerce_api(seat):
      return {
          'name': seat.name,
          'currency': seat.currency,
          'price': seat.price,
          'sku': seat.sku,
          'bulk_sku': seat.bulk_sku,
          'expires': seat.expires.isoformat() if seat.expires else None
      }