docs: def _login(session):
      """Login to Fedex Delivery Manager."""
      session.get(LOGIN_REFERER)
      resp = session.post(LOGIN_URL, {
          'user': session.auth.username,
          'pwd': session.auth.password
      }, headers={
          'Referer': LOGIN_REFERER,
          'X-Requested-With': 'XMLHttpRequest'
      })
      if resp.status_code != 200:
          raise FedexError('could not login')
      data = resp.json()
      if not data['successful']:
          raise FedexError(data['errorList'][0]['error']['message'])
      _save_cookies(session.cookies, session.auth.cookie_path)