input: Write a function to generate a random string of length 6 using lowercase letters.
  import random
  import string
  
  def random_string(length):
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))
  
  assert len(random_string(6)) == 6
  assert isinstance(random_string(6), str)
output: def random_string(length):
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))