Unsoundness relating to static fields

[marcoeilers]

The following program verifies, but should not:

from nagini_contracts.contracts import *

class Parent:
    x = 100


class Subclass:
    y = 200

    def __init__(self) -> None:
        self.x = 100
        Ensures(Acc(self.x))


def main() -> None:
    instance = Subclass()
    Assert(instance.y == 200)
    Subclass.y = 300
    Assert(instance.y == 200)

Found by @edgarsvitolins.