Add Final to typing_extensions

[ilevkivskyi]

This is a runtime counterpart of an experimental feature added to mypy in python/mypy#5522

This implementation just mimics the behaviour of ClassVar on all Python/typing versions, which is probably the most reasonable thing to do.

[JukkaL]

If this will be moved to typing, I hope there is a way to share code with the implementation of ClassVar. This is a lot of code for a relatively minor feature, but it's less of a problem if most of it goes away in the long term.

[ilevkivskyi]

In Python 3.7 it will be literally one line (not counting the docstring). But the point of typing_extensions is to be compatible with all versions of typing.

[msullivan]

This seems reasonable. At least, the newer the version being hooked into, the more reasonable it seems.

[ilevkivskyi]

OK, if there are no suggestions, I will merge this later today. We can release a new version of typing_extensions later this week.