Implement json.pyi for Python's stdlib

[brettcannon]

See python/typing#182 for the discussion. The resulting type of things should be JSONType = t.Union[str, int, float, bool, None, t.Dict[str, t.Any], t.List[Any]] (which should end up in Python 3.6: http://bugs.python.org/issue26396).

[gvanrossum]

Feel free to add this to the existing stdlib/3/json.pyi stub. Technically it should probably go in the 3.6 version -- you'd have to create a 3.6 subdir, copy json.pyi into it, and then add JSONType there. But we're not very strict about these things, and lots of other modules in stdlib/3 have features that aren't available in all versions from 3.2--3.6. At runtime people can use a trick like

if False:
    from json import JSONType

This way they can use JSONType and still run their code under 3.2--3.5. This would actually suggest that it's better not to introduce a separate 3.6 version. We should also add it to the 2.7. version.

[JelleZijlstra]

The json stubs in typeshed are fine and the typing issue for adding JSONType was closed with no action, so I don't think there's anything to do here. (Feel free to reopen if you disagree.)