docs: def _login(session):
      """Login.
  
      Use Selenium webdriver to login. USPS authenticates users
      in part by a key generated by complex, obfuscated client-side
      Javascript, which can't (easily) be replicated in Python.
      Invokes the webdriver once to perform login, then uses the
      resulting session cookies with a standard Python `requests`
      session.
      """
      _LOGGER.debug("attempting login")
      session.cookies.clear()
      try:
          session.remove_expired_responses()
      except AttributeError:
          pass
      try:
          driver = _get_driver(session.auth.driver)
      except WebDriverException as exception:
          raise USPSError(str(exception))
      driver.get(LOGIN_URL)
      username = driver.find_element_by_name('username')
      username.send_keys(session.auth.username)
      password = driver.find_element_by_name('password')
      password.send_keys(session.auth.password)
      driver.find_element_by_id('btn-submit').click()
      try:
          WebDriverWait(driver, LOGIN_TIMEOUT).until(EC.title_is(WELCOME_TITLE))
      except TimeoutException:
          raise USPSError('login failed')
      for cookie in driver.get_cookies():
          session.cookies.set(name=cookie['name'], value=cookie['value'])
      _save_cookies(session.cookies, session.auth.cookie_path)