Add support for graphene v3

[sjdemartini]

Currently this library only supports graphene-django v2 and thus graphene v2, but we should support the newer graphene v3 as well, like some other graphene third-party projects have done.

Granted, the graphene-django library https://github.com/graphql-python/graphene-django has not yet released v3 (only beta versions, see graphql-python/graphene-django#705 for tracking of this), but some folks are using that already and there should be a release of this package that supports it.