input: Write a function to automate the login process on a given website using Selenium in Python. The function should initialize a web driver, navigate to the target website, locate and fill in the username and password fields, and then submit the login form. The function should also print a message confirming a successful login and then close the driver.
  assert login_automation('http://www.example.com', 'username', 'password')=='Login successful!'
output: def login_automation(url, username, password):
      from selenium import webdriver
      from selenium.webdriver.common.by import By
      from selenium.webdriver.common.keys import Keys
      
      driver = webdriver.Chrome()
      driver.get(url)
      
      username_field = driver.find_element(By.NAME, 'username')
      password_field = driver.find_element(By.NAME, 'password')
      
      username_field.send_keys(username)
      password_field.send_keys(password)
      password_field.send_keys(Keys.RETURN)
      
      print('Login successful!')
      driver.quit()
      return 'Login successful!'