input: Write a function to display a digital clock in terminal.
  assert isinstance(display_digital_clock(), type(None))
output: def display_digital_clock():
      import time
      import os
  
      try:
          while True:
              os.system('cls' if os.name == 'nt' else 'clear')
              current_time = time.strftime("%H:%M:%S")
              print(current_time)
              time.sleep(1)
      except KeyboardInterrupt:
          pass