input: Write a python function to generate a username consisting of 10 random pairs of letters and digits.
  assert len(generate_username()) == 20
  assert generate_username() != generate_username()
  assert all(char.isalnum() for char in generate_username())
output: import random
  import string
  
  def generate_username():
      return ''.join(random.choice(string.ascii_letters + string.digits) for _ in range(20))