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)