Implement __hash__ for GraphQLError

[andrew-humu]

Fixes #34

This seemed like the most minimal implementation that might be safe, but it's definitely possible there's some context I'm missing. I also claim no mastery of python!

[Cito]

Thank your for providing this PR. Hashing only original_error assumes that one original error cannot cause two different GraphQLErrors, which is generally true, and that it is always set, which is not always the case. It would be better to derive the hash from all slotted attributes.

But I think the best and most performant solution is to simply use the default hash method for exceptions, based on the id of the instance. GraphQLError lost that default hash capability when it got a custom __eq__ method, but the default hash method can be restored by setting __hash__ = Exception.__hash__. Can you check if that works for you and change the PR accordingly?

[andrew-humu]

Your reasoning makes a lot of sense to me. I pushed a commit — is this what you had in mind?

[Cito]

Nice, thank you.