input: Write a python function to generate a random string of lowercase letters with a specified length.
  assert len(random_string(4)) == 4
  assert len(random_string(8)) == 8
  assert isinstance(random_string(5), str)
output: import string
  import random
  
  def random_string(length):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))