Unhashable objects cannot be used with `once_per_instance`

[aebrahim]

Demonstration of the issue:

import once

class Foo:
    def __init__(self, a: int):
        self.a = a

    # defining this without __hash__ make unhashable
    def __eq__(self, other) -> bool:
        if isinstance(other, Foo):
            return self.value == other.value
        return NotImplemented

    @once.once_per_instance
    def b(self) -> int:
        return self.a

a = Foo(a=1)
a.b()

This gives TypeError: unhashable type: 'Foo'