Move Protocol, runtime_checkable, Final, final, Literal, and TypedDict to typing

[ilevkivskyi]

Fixes #648

This essentially just moves a bunch of code and tests from Python 2 version of typing_extensions to Python 2 version of typing. We don't need to copy the definitions in the former, and can just re-export them from typing, since there is not built-in typing module on Python 2, so that we can always assume the latest version of typing (we will just need to release them in right order, typing first, and then update dependency in typing_extensions).

Notable changes apart from the code move:

• Like on Python 3, keep runtime in typing_extensions as an alias for runtime_checkable for backwards compatibility (the former is not included in __all__)
• Update GenericMeta.__new__ and GenericMeta.__getitem__ instead of almost identical overrides in _ProtocolMeta.

[gvanrossum]

Where's the dependency on a version of typing.py specified in typing_extensions?

[ilevkivskyi]

Where's the dependency on a version of typing.py specified in typing_extensions?

In typing_extensions/setup.py.

[ilevkivskyi]

install_requires = []
if sys.version_info < (3, 5):
    install_requires.append('typing >= 3.6.2')

[gvanrossum]

Skimmed, LG.