[Datetime-SIG] Another round on error-checking

[Tim Peters]

[Tim]
>> The easiest way out of this particular puzzle is, I believe, to say
>> that two datetimes identical except for `fold` do _not_ compare equal.
>> `fold` breaks the tie in the obvious way (the one with fold==1 is
>> "greater").

[Alex]
> I am afraid you are right, but proving that we will not break naive (fold
> unaware) programs will be harder in this case.   Let me think some more
> about this.
>
> Meanwhile, would you see any problem with not(x - y) not implying x == y?

Which is another puzzle :-(  It's very intentional now that

    dt1 == dt2 if and only if dt1 - dt2 == timedelta(0)

Here's a related puzzle, if comparison used `fold` to break ties:

    y = x + timedelta(0)

If x had first=1, y will have fold=0, and then x != y.

In all, maybe it's better to leave __hash__ slightly broken.