Support assertion style type guards in exhaustive branch analysis

[KotlinIsland]

def func(a: object, b: bool) -> None:
    if b:
        assert isinstance(a, int)
    else:
        assert isinstance(a, str)

    reveal_type(a)  # note: Revealed type is "builtins.object"

a should be narrowed to int | str but it is instead not narrowed at all.

[hi-ogawa]

Considering the behavior of the example below, "narrowing" is happening in some sense, but possibly mypy's join type is producing join(int, str) = object.

# test1.py

class P:
    pass


class C1(P):
    pass


class C2(P):
    pass


def f(a: object, b: bool) -> None:
    if b:
        assert isinstance(a, C1)
    else:
        assert isinstance(a, C2)

    reveal_type(a)  # note: Revealed type is "test1.P"

[brianschubert]

This was implemented in #18538, which should release with v1.16.0