Don't use mutable keys

from https://gist.github.com/astynax/0f759f1527d6bdcf3d26c48837c59eb8

>>> class Point:
...     def __init__(self, x, y):
...         self.x, self.y = x, y
...     def __hash__(self):
...         return hash((self.x, self.y))
...     def __eq__(self, other):
...         return (self.x, self.y) == (other.x, other.y)
...     def __repr__(self):
...         return "Point({},{})".format(self.x, self.y)
...
>>> p = Point(10, 20)
>>> d = {p: 'a'}
>>> d
{Point(10,20): 'a'}
>>> p.x += 1
>>> d[p] = 'b'
>>> d
{Point(11,20): 'b', Point(11,20): 'a'}
>>> ks = list(d.keys())
>>> ks[0] == ks[1]
True
>>> hash(ks[0]) == hash(ks[1])
True