input: Write a python function to generate a random MAC address.
  assert len(gen_mac_address().split(':')) == 6
  assert all(0 <= int(segment, 16) <= 255 for segment in gen_mac_address().split(':'))
  assert isinstance(gen_mac_address(), str)
output: import random
  
  def gen_mac_address():
      return ':'.join(f'{random.randint(0, 255):02x}' for _ in range(6))