Added Static typing to graphql-core.
It should help to anyone using the library so they can have autocompletion and static type analyisis on their code.

• Use pyannotate to automatically annotate all types
• Fixed automatic imports from pyannotate
• Fixed flake8
• Use black to autoformat all the files
• Correct annotations, making mypy pass (and eventually pyre-check)
• Add mypy checks to Travis
• Check one by one all files and declarations to make sure they are properly annotated