用Python定义一个people类，包含属性name，sex。为people派生一个之类Eemployee类，为其增加两个属性age和grade。grade分等级A，B，C三个等级。

class People:
    def __init__(self, name, sex):
        self.name = name
        self.sex = sex
        
class Employee(People):
    def __init__(self, name, sex, age, grade):
        super().__init__(name, sex)
        self.age = age
        self.grade = grade
        
    def __str__(self):
        return f"Name: {self.name}, Sex: {self.sex}, Age: {self.age}, Grade: {self.grade}"

示例：

e = Employee("Tom", "Male", 30, "A")
print(e)
# 输出：Name: Tom, Sex: Male, Age: 30, Grade: A