[Ndiff] Fix DeprecationWarning under python -3

Classes that inherit __hash__ and comparison functions like __eq__ (e.g.
from object) but only override one of them will break under Python 3.
This is because a class shouldn't use one criterion for equality and a
different one for hashing. Explicitly discarding the inherited __hash__
method disables this warning and makes the class unhashable (not a
problem in this case).