Update to the latest git:
    
    - 2.7 does not have lru_cache()
    - Add keyword args to stdlib's json.pyi
    - Add Generator to 2.7/typing.pyi.
    - Make Reversible covariant.
    - Fix Reversible.__reversed__() return type.
    - Add flush keyword arg to print()
    - Move contents of builtins/* to stdlib/*. This simplifies finding stubs.
    - Remove outdated things from README.
    - Make deque inherit from MutableMapping (with difficulty).