input: Write a python function to generate an access token and its expiration time.
  assert isinstance(authenticate('user1', 'pass1')[0], str)
  assert authenticate('user2', 'pass2')[1] > datetime.utcnow()
  assert len(authenticate('user3', 'pass3')[0]) == 32
output: import secrets
  from datetime import datetime, timedelta
  
  def authenticate(username, password):
      token = secrets.token_hex(16)
      expiration_time = datetime.utcnow() + timedelta(hours=1)
      return token, expiration_time