用户登陆（三次机会重试）

i = 0
while i < 3:
    username = input('please input your username: ')
    password = input('please input your password: ')
    if username == 'test' and password == '123456':
        print('登录成功！')
        break
    else:
        print('登录失败，请重新输入用户名与密码。。。')
    i += 1