Upgrade to graphene v3

[DoctorJohn]

(Related issue #154 )

Hi, I just spend my sunday porting this package to graphene v3 and graphql-relay-py v3. I never used this package (nor mongo), so I didn't have the chance to test my changes with an real project.

There is one problem we need to think about: dataloaders and the promise library

There is a dataloader test which I only got green by resolving the dataloaders promise myself. I guess before graphene resolved it, but as far as I can tell graphene dropped support for promise dataloaders.

One more thing to consider is that graphene now requires Python >=3.6. (CI also tests 2.7, 3.4 and 3.5, thus CI fails).

Additional infomation can be found in the graphene v3 release notes and the graphene v3 issue: graphql-python/graphene#1127

[KingDarBoja]

This should be released as alpha / beta version of graphene-mongo v3 to keep everything on the "same" semantic versioning.

[aryaniyaps]

Hey @DoctorJohn The promise abstraction for python v2 was dropped because python3 comes with better asyncio support.
Hence, I think that a drop-in replacement would be this package. Have you checked this out?

I might as well as fix the broken tests with this package.

https://github.com/syrusakbary/aiodataloader

[memark]

Any updates on this? This is currently one of only two remaining blockers for releasing Graphene v3.

[DoctorJohn]

Looks like support was added in #172 thus I'm closing this PR.