assert and isinstance type inference

[JukkaL]

This code should be accepted by mypy:

def f(x: Union[int, str]) -> None:
    assert isinstance(x, int)
    x + 1  # type of x is int, so this should be okay

The type type checker should know about type assertions.

[JukkaL]

This could sometimes be used as an arguably more Pythonic replacement for casts.

[gvanrossum]

I found a bunch of occurrences of this in our server code; bumping priority to 0.3.2.