Let's add typing_extensions module to PyPI in addition to Mypy-specific mypy_extensions.

Currently new stuff in typing appears in mypy_extensions even though most of the new typing features are not Mypy-specific.

In order to make new typing features available to 1) previous Python versions, 2) all type checkers I would suggest to move these features to typing_extensions.

The good candidates for including in typing_extensions could be:

• NoReturn (apparently already in PEP 484)
• Protocol (draft PEP 544)
• Argspecs?
• TypedDict?