input: Write a python function to generate a unique 6-character string consisting of uppercase letters and digits.
  assert len(get_unique_string()) == 6
  assert all(c in string.ascii_uppercase + string.digits for c in get_unique_string())
output: import random
  import string
  
  def get_unique_string():
      return ''.join(random.choices(string.ascii_uppercase + string.digits, k=6))