Disallow isinstance of generic expressions

Now that runtime generics are allowed (thanks to #2302), we should prevent people from making calls like:

isinstance(["foo"], List[int])

because it won't behave as expected. I.e. in this case, it will return True. (First instance of user confusion appeared in #2369.)

Latest version of typing.py raises TypeError for things like this. Only unsubscripted generics etc. are allowed with instance/class checks.

But mypy still doesn't give an error!

Oops, sorry. Classified as bug for milestone 0.5. (Since it's a runtime error I don't think it's a very big deal if mypy doesn't catch it.)

This is fixed now, mypy says

error: Parameterized generics cannot be used with class or instance checks

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