请把下面的Student对象的gender字段对外隐藏起来，用get_gender()和set_gender()代替，并检查参数有效性

class Student(object):
    def __init__(self, name, gender):
        self.__name = name
        self.__gender = gender
    def set_gender(self,gender):
        self.__gender =gender
    def get_gender(self):
        return self.__gender

# 测试:
bart = Student('Bart', 'male')
if bart.get_gender() != 'male':
    print('测试失败!')
else:
    bart.set_gender('female')
    if bart.get_gender() != 'female':
        print('测试失败!')
    else:
        print('测试成功!')