input: Write a python function to check account balance from a CSV file and send a notification if the balance is below the threshold.
  assert check_account_balance(123456, 1000) == True
  assert check_account_balance(654321, 1000) == False
  assert check_account_balance(123456, 500) == False
output: def check_account_balance(account_number, threshold):
      import csv
  
      with open('account_balances.csv', mode='r') as file:
          csv_reader = csv.DictReader(file)
          for row in csv_reader:
              if int(row['account_number']) == account_number:
                  balance = float(row['balance'])
                  return balance >= threshold
      return False