Initial JSON Type Support in Python

Because typing system in Python lacks of recursive type, we can not annotate JSON-like data for type hints. Recently, There is a progress that an experimental JSON type has been implemented under _typeshed lib.

Try it like:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from _typeshed import JSON

Of course, we still need to wait offical JSON type support.

Define a JSON type #182

Update 2022-06-22: The PR in typeshed still can not be merged.