Sync typing.py with upstream.

(Upstream is https://github.com/python/typing)

- Add TYPE_CHECKING (false at runtime, true in type checkers) (upstream #230).
- Avoid error on Union[xml.etree.cElementTree.Element, str] (upstream #229).
- Repr of Tuple[()] should be 'Tuple[()]' (upstream #231).
- Add NewType() (upstream #189).