assert and isinstance type inference

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.

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

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

Get type information from isinstance assertions

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