nixops.util.ImmutableValidatedObject: Allow passing an instance of self

[adisbladis]

This is useful for plugins or others that want to extend a class.

Consider the following code

from nixops.util import ImmutableValidatedObject

class A(ImmutableValidatedObject):
    x: int

class B(A):
    y: int

a = A(x=1)
B(a, y=1)  # Type checks both x and y