Complete the Mint and Coin classes so that the coins created by a mint have the correct year and worth. Each Mint instance has a year stamp. The update method sets the year stamp to the current_year class attribute of the Mint class. The create method takes a subclass of Coin and returns an instance of that class stamped with the mint's year (which may be different from Mint.current_year if it has not been updated.) A Coin's worth method returns the cents value of the coin plus one extra cent for each year of age beyond 50. A coin's age can be determined by subtracting the coin's year from the current_year class attribute of the Mint class.

以下是Mint和Coin类的完整代码：

class Mint:
    current_year = 2021

    def __init__(self, year):
        self.year = year

    def update(self):
        self.year = self.current_year

    def create(self, coin_class):
        return coin_class(self.year)


class Coin:
    def __init__(self, year):
        self.year = year

    def worth(self):
        age = Mint.current_year - self.year
        cents = self.cents
        if age > 50:
            cents += (age - 50)
        return cents


class Quarter(Coin):
    cents = 25


class Dime(Coin):
    cents = 10


class Nickel(Coin):
    cents = 5


class Penny(Coin):
    cents = 1

在上述代码中，Mint类有一个current_year类属性和一个year实例属性。update方法用于将实例属性year更新为current_year类属性。create方法接受一个Coin子类，返回一个带有Mint年份的该子类的实例。

Coin类有一个year实例属性和一个worth方法。worth方法返回硬币的价值（以美分为单位），并根据其年龄增加额外的一分钱。Coin的子类Quarter、Dime、Nickel和Penny分别定义了硬币的价值（以美分为单位）。

注意，Mint.current_year是一个类属性，因此每个Mint实例都可以访问它。