Add support for Python 3.8 in typing_extensions

[ilevkivskyi]

Fixes #643

(Use ignore whitespace option to review this, since it mostly just indents a bunch of stuff.)

PR summary:

• Update Travis config to run on a bunch of newer Python versions (including 3.8-dev).
• Only run typing tests on Python 3.6 or older, starting from Python 3.7 development of typing moved to CPython repo, where it was significantly reworked. We still run typing_extension tests on all versions.
• Update repr() tests in typing_extensions to conditionally expect correct module name.
• Add TypedDict to __all__.
• Use Final, Literal, etc. from typing if those are available (as we do for other things).
• Rename @runtime to @runtime_checkable in typing_extensions (I think we can keep @runtime in typing_extensions as an alias for backwards compatibility).

Note this only updates Python 3 version of typing_extensions. There will be a corresponding update for the Python 2 version of typing_extensions as part of #648

[gvanrossum]

OK.