Travis uses tox

[dan98765]

Travis runs various tox environments to do testing.

In addition:

1. Drop py33 support
2. Do not run flake8 or isort as part of tox; those are taken care of via pre-commit;
   a. isort added in A possible minor bug in ExecutionContext.complete_value graphql-core#187
   b. flake8 added in Can you inject fields to an operation with a Visitor? graphql-core#191

[dan98765]

py27 and pypy are failing due to graphql-python/graphql-core#189 - I think it makes sense to fix that as a separate branch then circle back to this one when it's resolved to prevent this branch getting even more enormous.

edit fix is over at graphql-python/graphql-core#190

[dan98765]

@syrusakbary I've resolved merge conflicts with latest master. What do you think of merging this in? I think it makes the travis behavior easier to understand and also contributers who run tox will end up getting the same behavior as travis so no surprises. It'll also match graphene with graphql-python/graphene#737

[syrusakbary]

I'm working on adding static type checking. Since it will probably merge conflicts between this two branches (as this is also formatting some code), I will wait to merge.

graphql-python/graphql-core#193

[syrusakbary]

If you want to rebase from master I will happily review this PR again :)

[dan98765]

@syrusakbary OK will do! I still think having pre-commit (which runs flake8 and other checks) run as part of travis will help maintain good code quality. I'll see about updating this one tonight.

[dan98765]

@syrusakbary this is ready for a review.