【398】COMP9021 - Polynomial

构建 Polynomial 类，实现 +, -, *, / and +=, -=, *=, /=

参考：如何用python编程求解二元一次方程组。如x+y=3;x-y=1
参考：python对重载运算符的限制
参考：python:自定义对象的打印

operator	overwrite

+ | \__add__ - | \__sub__ * | \__mul__ / | \__truediv__ += | \__iadd__ -= | \__isub__ *= | \__imul__ /= | \__itruediv__ 字符串打印 | \__str__

改写举例：

def __add__(self, other):
    ...
    return Polynomial(self.ex + other.ex)

def __iadd__(self, other):
    return Polynomial(self.ex) + Polynomial(other.ex)