Drop Python 3.5

[mirumee]

Drop Python 3.5

[hugovk]

To test Python 3.7 on Travis CI, a little workaround is needed: travis-ci/travis-ci#9815.

[rafalp]

Based on feedback we are getting, the strongest argument for dropping 3.5 would be moving to GraphQL-Core-Next, thus letting folk experiment with async resolvers.

Originally we've started out with py3.5 because we were eyeing possible use in the Saleor and they have 3.5 on supported python versions list, but I've discussed this with @maarcingebala recently and Saleor will stick to Graphene for some time now, so we can cross this off the concerns list.

The next core was still work in progress back when we were making a push for query execution milestone, but now that its out, we could hop to it. Only gotcha here is that we'll have to create a shallow fork that namespaces it to graphql_next to support projects that will want to try it out side by side to GraphQL.