input: Write a function to generate a random string of a specified length.
  assert len(get_random_string(8))==8
  assert len(get_random_string(15))==15
  assert len(get_random_string(0))==0
output: def get_random_string(length):
      import random
      import string
      return ''.join(random.choices(string.ascii_letters + string.digits, k=length))