input: Write a function to create a user with a hashed password.
  assert create_user('alice', 'password123')['username'] == 'alice'
  assert 'salt' in create_user('alice', 'password123')
  assert 'hashed_password' in create_user('alice', 'password123')
output: import hashlib
  import os
  
  def create_user(username, password):
      salt = os.urandom(16)
      hashed_password = hashlib.pbkdf2_hmac('sha256', password.encode(), salt, 100000)
      return {
          'username': username,
          'salt': salt.hex(),
          'hashed_password': hashed_password.hex()
      }