input: Write a function to generate a random string of length 8 containing alphanumeric characters.
  assert generate_random_string(8)=='a1b2c3d4'
  assert generate_random_string(8)=='z9y8x7w6'
  assert generate_random_string(8)=='q3r4s5t6'
  assert generate_random_string(8)=='j7k8l9m0'
  assert generate_random_string(8)=='p5o4n3m2'
output: import random
  import string
  
  def generate_random_string(length):
      return ''.join(random.choices(string.ascii_letters + string.digits, k=length))