Allow Q objects to be hashable, so that they can be used in sets

[Keryn Knight]

Given:

>>> x = set()
>>> x.add(Q(a=1))
>>> x.add(Q(a=1))

I was *expecting* that the x would end up with {<Q: (AND: ('a', 1))>} but instead it ends up with {<Q: (AND: ('a', 1))>, <Q: (AND: ('a', 1))>}

For Q objects to work in sets (and as dictionary keys I guess) the class would need to implement __hash__ and __eq__ methods I think. This is slightly complicated by the fact it consumes all *args, **kwargs so doesn't necessarily have a concrete set of values to easily equality-check (and they could feasibly be nested, and in a different order [eg: Q(a=1, b=2) and Q(b=2, a=1))

[Simon Charette]

Confirmed this is a duplicate of #28629 which is fixed in Django 2.1.