input: Write a function to generate a random string of length 8 made of uppercase letters and numbers. The function should return the generated string.
  assert generate_random_string()==some_string
  assert generate_random_string()==some_other_string
  assert len(generate_random_string())==8
  assert any(c.isdigit() for c in generate_random_string())==True
  assert any(c.isupper() for c in generate_random_string())==True
output: import random
  import string
  
  def generate_random_string():
      return ''.join(random.choices(string.ascii_uppercase + string.digits, k=8))