Many changes from the upstream repo (https://github.com/python/typing).

Many changes from the upstream repo (https://github.com/python/typing).

This syncs to rev 7b43ada77821d23e55e3a4b35f6055a59b9e1ad7 there.

Summary:

- Add typing.DefaultDict (as a generic variant of collections.defaultdict).

- Use collections.Reversible if it exists (only relevant for Python 3.6).

- Revamped generic class behavior to conform to updated PEP 484.

- Improve speed of Generic.__new__.

- Make sure __init__ is called for new Generic instances. Fix issue #26391.

- Refactor async support to be compatible with 3.2, 3.3, 3.4.

- Remove 'io' and 're' from __all__ (they still exist, just not
  included by "import *"). Fix issue #26234.

- Change @overload -- you can now use it outside stubs (you still
  cannot call the decorated function though).